Question: When is it acceptable to roll need on items?
My Vote: Whenever it is an upgrade for your current gear and spec.
If the item is for the class you are currently playing and an upgrade, then the roll of 'Need' is perfectly fine in my opinion. If the gear will be an upgrade for a companion on the current character, roll 'Greed' or ask first if you can 'Need' for a companion. Lately I've not bothered asking to roll 'Need' for companion gear, I choose to just roll 'Greed' to keep the pace of the FP or OP moving along.
It really seems you really just have to adapt to the situation. If you notice a player waiting for everyone roll then rolls 'Need' or simply rolling 'Need' on everything, then perhaps the group should react to the situation. There probably isn't a cookie cutter solution for PUGs.