Rule of thumb. Wait for all players to greed, if all before you have done so - THEN ask if it's okay, if you want the purple crystal... or for a comp, if ANY of the group says no... greed like everyone else and move on.
Heck, I feel bad when I run greeds for gear I can't use - to the point I'll offer it away to groupies, if someone wants it... they get it.
I personally dislike even when people ask to need for companions, since that's what greed is for - your main can't use it, but you need it for companions/mods/etc. If you don't need it at all, just PASS.