With boxing friends(the zerg) we usually either decide who gets something based on who it's the biggest upgrade for or everyone gets one roll. With non-boxing friends doing leveling 5-mans(I generally 3/4 box with one or two friends), I usually pass on all loot that one of my friends needs since it's it'll help them more, but I was told to roll on all of my chars when I do. In raids at 80 I roll on all chars that would normally roll. Usually I don't tell pugs that I'm boxing though. Only one person in my raiding guild brought something up and she was mostly ignored, although with the characters I was using at the time they wouldn't be rolling on the same item anyway(tank/dps 2-box). The only times I could see only rolling on one character is if only one of your characters needs it, if you're being carried and trying not to leech much, or if you're in a guild where you have a rule that mains get loot before alts in which case you'd only roll on your main.