Just my 2 cents here: On tier sets/class exclusive pieces, show some consideration for your fellow (insert class here)s and only roll once. No one wants to be the soloboxed shaman having to roll against 5+ shamans all played by one guy for every piece of relevant loot. On the wider user-base peices, roll on all of them.

Of course, I suppose the lack of a hard and fast rule on that would confuse the uninitiated. They'd probably get pissed you were rolling on just one character for some pieces and all your characters for others.

Damn I'm glad loot is up to my sole discretion.