Within a couple of days, there is no respawn. Respawn now takes 30 ingame days, which is longer than in previous ES games.
You even get a "cleared" marker on the map when you eradicated an area of all its inhabitants for the duration.
However, when you have not fully cleared sites, enemies may respawn. I experienced this myself at some fort which name I can't remember right now. I killed 3 of the guys outside, went into a tower inside the fort, came out again and they respawned.
By the way, storing stuff inside containers which do not explicitly belong to you (e.g. stand in your house) doesn't seem to be safe. At least one of the loading screens explicitly says that storing anything in "wild" containers does not guarantee for it to be in there when you look for it the next time.
One way to find a dungeon which has respawned enemies inside is to accept one of the various kill quests that you can find (from innkeepers, the Dark Brotherhood, etc.), which will lead you to you to these dungeons. These procedurally generated kill-quests will send you to dungeons where the difficulty inside matches your level, which means that taking one of these at higher levels will have a higher likelyhood of taking you to a more difficult respawn-dungeon.
Summoned daedra disappear anyway when they die. You can't get fire salts from a summoned fire atronach, only from wild atronachs that leave a corpse.
So banishing summoned creatures does cause them to disappear, but they wouldn't have been able to drop anything anyway, so there's no reason to avoid doing so.
Best Answer
Bodies do not persist forever. Eventually they will be removed.
I was just beginning to think that the dragon skeleton at the College would be around forever, but when I visited it now it was gone.
Similarly, when I killed Belethor, I was surprised to see that his body had been cleaned up rather quickly.
Corpses tend to stay around longer than you would ever have need for them (I presume to give you adequate chance to loot), but they do disappear eventually, no matter their source.