Since there are many people getting 5 day bans from dungeons, I would guess that even the normal group loot mechanics can be a target for ninjas.
For example, if a hunter needs on an off-hand orb that is meant for healers, then I would regard that as a ninja loot. And yes, hunters can need on those (this exact thing happened for me).
I would never need on something for an off-spec or something that I don't need in a dungeon. The only exception is when everyone greeds on it and I want it for an off-spec, but I even ask the group before doing that. Anything else than that, I would regard as a ninja, how else do people get 5 day bans all the time?