Let me start with two assumptions in answering your question.
The party has 5 characters and they are 3rd level each. I am doing this deliberately to help you focus on the mechanics of the system and then take those mechanics back to your group.
Standard XP rewards are basically setup to work on an 8-10 encounter basis. This means that within 8-10 encounters a typical party of 5 characters will go up 1 level. The treasure parcel system is a way to parallel this. Remember that encounters are NOT the number of rooms/areas explored or passed through they are the number of fights, traps, skill challenges or other events that require characters to worry about their health.
DMG 1 Page 126:
During the course of gaining that level, expect a group of five characters to acquire four magic items ranging in level from one to four levels above the party level. In addition, they should find gold and other monetary treasure equal to the market price of two magic items of their level.
A 3rd level party would receive 4 magic items (levels 4, 5, 6, 7) and about 1350gp in monetary items. Looking at the parcel system as shown on page 126 of the DMG1 you will see this laid out in exactly that fashion. 10 parcels nice and neat. There is a guide on page 125 for adding or removing gp value and magic items based on actual party size. The PHB1 on page 223 has nice table of the gp vaule of magic item by level.
Now here is the trick. While this is a good guide of what to give out it is not a hard-set-in-stone law. You can combine, rearrange, add to or delete from this guide list to adjust for what makes sense for your campaign. You can give out 1 parcel per room or all at once or anything in between. Do what makes sense. Fighting 8 rooms of humanoids and 1 room with a dragon and all the treasure is in the dragons lair might just make sense.
As for how to pick magic item for characters I use three methods that overlap.
Special items I want in the campaign. This might be unique items I have created or special cool things I think that a character would use that they do not know about. (Not everyone has all the books, magazines and supplements - except me of course).
Character wish lists. Every player is required to give me a list of 10 items that they would like to see in their characters possession. It must contain magic items for each level starting at 1 level higher than their current and going to 5 levels higher. Limit 10 items on the list. They do this as part of leveling up. They have to include source and page number too.
Player pick. About once every 2 levels or so I let the player pick a magic item instead of having a specific item in a parcel. That item is only 2-3 levels higher than their current character level. This allows the player to 'customize' a magic item selection to fit their goals, skills or feats that they have chosen, and takes the pressure off of me for that area.
I have found that using the parcel list a basis and then tweaking it to fit the campaign really simplifies my game prep time. Using the 3 methods of magic item selection has given the party good balanced selection items that they can use and like to use.
NOTE: Be sure to rotate the highest level items in your parcels to be given out equally among the party characters.
Choose whatever looks appropriate, plus a few flavourful items for theme, from TreantMonk's Guide, and Bunko's Bargain Basement. Choose flavourful items that will prove to be macguffins for your campaign.
He explores all the magic items someone would have. An 18th level wizard will have 3 categories of items. "Junk I collected," "Stuff that's applicable to what I do," "Awesome quest MacGuffins."
We're looking at "Junk I collected" to be easily accessable, and everything else to be hidden away in quest locations.
In a spreadsheet, set aside 440,000 GP. This is an 18th level wizard's wealth by level. In contrast, the party wealth should be around 20k. The handbook lists most prices, so simply note down the item and how much it costs until you've spent about half your gold, fill the rest up with weapons of legacy or other notable items. Then, take 10% of the items (I'd say chosen randomly) and have them "out in the lab." Which the party, if it finds them, can just pick up. Everything else is stored in the series of interlinked vaults that the party can raid of they want to.
The Normal Effective Wizard will have, on him,
Ring of Feather Fall, Ring of Enduring Arcana, Headband of Intellect (+6), Amulet of health (+4), Metamagic Rod of Quicken (lesser), Metamagic Rod of Extend (lesser), Metamagic rod of Silent Spell (lesser), Heward's handy haversack, Heward's fortifying bedroll, a (good) metamagic rod.
This is his "walking around" equipment. Plus a "travelling" spellbook (http://rpg.stackexchange.com/a/18374/760) of useful spells.
We'll assume that he has dropped at least 100k into infrastructure (which is, itself, loot). In fact, a home base could make an excellent legacy, and a touchstone to your campaign. Give it a few semi-friendly ghosts, a horde of semi-magical servants, and suddenly it has character and (to a large degree) safety.
Grab a few items from Tome of Magic as curiosities (drop in a vestige tooth or two, just for sheer WTF, and some of the items in the shadow area), and top it off with 5 or so level 10 loot rolls, representing the quite literal loot he's taken off people.
Best Answer
Don't worry about it. Render the treasure as per treasure parcels, and give the rest as a plot token which represents the rest of the treasure in the narrative of your game.
Dump most of the treasure parcels for the level at the end of the encounter. That means the dragon will be sleeping on about ninety thousand gold, not counting magic items. Beyond that, money in late paragon is almost entirely pointless with magic item rarity rules. Healing potions already cost as much as a small town.
What I'd recommend is, on top of focusing most of the level's treasure parcels on the hoard, say: "there's a lot more money here, I'll be giving you this level's money all in a lump, but because there's more than that, each of you get a 'plot token.' These plot tokens represent you spending massive amounts of money to affect some change onto the world. How do you want to spend them?"
By treating the hoard by treasure rules, you continue to maintain the character-required levels of wealth and other nonsense. By giving most of the hoard as "plot tokens" you explicitly acknowledge that "Hey, the characters have done something special here" and reward them by giving them a stake in the world.
By late paragon, most characters likely have pocket kingdoms (or equivalent) and are heading into the multiverse-spanning epic levels. Don't worry too much about "cash."
Example uses of plot tokens in the game that I played:
The beauty of most of these requests is that they inevitably lead to more adventures. Not only that, but the plot resources implicit in these things lead to fantastic backdrops of "As your city's army battles it out with the tainted creatures of the world around the iceberg, you head inside." This investment-in-world also gave the paragon-epic characters reasons to fight without needing to deal with the standard epic quest-giver problem.
If you absolutely must give treasure qua treasure beyond the 90k in gold, gems, and art, consider a "ritualized residuum" substance. With normal residuum, casters can only create common magic items. With this stuff, say because a dragon had been using it for a bed for umpteen years, it can be used to enchant uncommon items of the players choice. You can then make the magical crystals which contain the "ritualized residuum" rather bulky, which gives an endless floor of twilit sparkly magic rocks.
Given that (the never considered size) of gold pieces in earlier editions was 1.5 to 3 times the size of one of our 50 cent pieces ninety to a hundred and fifty thousand of them is... not too bad for a floor covering. Feel free to emphasise how many there are by not counting them but providing the means for a quick fermi calculation as to their quantity.