As Kaurie mentioned above, by asking people to go in with the class/job that they need the item for (which is obviously undergeared if they need a dungeon item) you are directly adding a burden on 3 other players.

I have been lucky that most of the groups I had would "greed" by default. And my default choice is always greed. The only cases where I would roll need are:
1. I am with a <60 class and I need a green/pink item for an immediate upgrade
2. it's a lv50 duty (esp. raids) and I need the item for glamour
3. someone in the party has rolled "need" on an item just because they could -I'll be damned if I let their greed score them anything for my job.