I switched from 2boxing to 5 boxing, mostly PvE oriented as well (PvP to fill in some gear holes while I collect stuff pretty much).

I run with loot on FFA and loot up everything with my main, I have one char that disenchants, another that tailors, another that just has lots of bag space for gray vendor junk etc. When my main guy fills up I pause for a moment and clear out my bags putting stuff on the right storage toons until I should hit town again (or they all fill up whichever is first).

Questing x5::
I avoid most collection quests if they aren't 100% drop rate items or close. The exception to this rule for me is if the collection quest starts or is in the middle of a quest chain that I am interested in completing.
I usually just grab quests with my main toon and share them with the other 4.

Turning in quests::
I don't have a synchronized mouse, so I just click on each screen through the quest complete dialog.