The best option is to go premade with people you know.
If you solo then yes you should ask people what builds are they using and what lanes they want to be in,people will often be ok and tell you how they plan to play the game,but of course there are times when you just get people that want to troll and there is nothing you can do about it.
Personally what i sometimes do when i enter solo queue is that i type in:"STOP DON'T LOCK IN" as soon as i enter queue,and people often listen and do not lock it after that,and then i tell them that we should build our team carefully and most people are ok with that.
Of course some people get rude about it and lock in with random champion and there is nothing you can do about them,either dodge queue or stick with them and hope for the best.
I would also mention that even if you get really weird team combination that you can still win if you're lucky.Even if your team may have crappy combination there is a chance that enemy team doesn't have good combination either or that they don't know how to play their champions so good.So there is always hope.
[Updated for Season 3]
The game is currently in flux (season 3 has only been out for a couple days), but with the boosted ambient gold gain, it appears you only need one gp/10. I've been building Philo Stone to great success. If you expect to get some significant assist-gold (you're confident in your kill lane, or you're Soraka and have a good eye for your ultimate), you could probably stand to buy a Kage's Lucky Pick, but I much prefer to only buy the one Philo Stone and start immediately after into my build (Sightstone, Shurelya's, Aegis -> Bulwark).
Again though, this is all pretty fresh. Your mileage may vary.
[NOTE: The answer below was provided for season 2. There were significant changes implemented for Season 3, and this answer is now out of date. It is provided here for history, but should not be followed.]
It depends. The rule of thumb is two (Philo Stone and Heart of Gold), and 98.355% of the games, you'll want those. If the game is going to absolute crap, teamfights started early and your team is getting routed, you may have to skip the second and start pouring that extra money into wards and an oracle's (counter to what you'd think, oracle's is a great purchase if you're falling behind and the laning phase is over).
I'd be very suspicious of my support if they got a third gp/5. The only real time I can see that being worthwhile is if you know it'll be a late, late game, and that you'll need to be a significant contributor in teamfights for damage.
Best Answer
The amount you return to fountain should always be minimized
You'll notice an experienced player returns to fountain much less often than a newer player. Less time at the fountain means more income.
We can break down the logic into periods of the game:
Laning
High level goals of laning phase:
Backing Tips:
Try to return after pushing your lane.
Make sure your opponent can't shove the lane and cause you to lose CS at the tower while you're returning
Try to return ONLY after you've saved enough to actually purchase something
What's the point to returning to heal, if you could have waited one more wave and purchased a big item to help?
Return if you need additional health or wards
This is tricky. Unless you feel like you're in absolute danger of dying to an attack or gank from the jungler then you do not need to go back.
Mid Game
High level goals of Mid game:
Backing Tips:
Don't just back if you're low on health or mana
Even if you're low on health, if you're knowledgeable of the enemy and have map awareness, you can still farm and gain gold until you have enough for an item. This is especially true if you have life steal.
Playing a mage with low mana? Knowing blue is up in 15 seconds can save you a trip and valuable time.
Don't back when you know an objective is close
I know you're dying to finish your Infinity Edge, but what's it worth when you're back at base and your team just died at dragon?
If you find yourself in a position where you can get significantly stronger, ask your team to stall an objective while you purchase. Make sure you have vision of the objective before backing.
Late Game
High level goals of late game:
Backing Tips:
Be sure to back with your team, or when you don’t put them at risk
I've seen a lot of games lost from one poor initiation late game. If you're not with your team, you increase the chance of having your team killed without you. Back when you feel you're not going to place your team in a position when you're at base, and they're getting caught.
Know timers for objectives
If baron is up in 10 seconds and the enemy knows you're backing, you better believe they're going to start baron knowing they have an advantage. As I mentioned earlier, if you MUST back, let your team know to stall, and do NOT let the other team know you're backing.
That's just the tip of the iceberg, honestly. It all comes down to intuition. Best of luck
-- Taijii