If you need it, why would you roll greed? If pretty much everyone can use them, sounds like they should need it too. Letting the group know is perfectly fine, but since you are equally entitled to the loot, and the need button is for items you need... well, I'll let you do the math.
Rolling greed on something you needed and then crying when someone else needed it and rolled need is something a 12 year old would do. (see what I did there?)
If an item you need drops, why would you roll greed then go buy one?
If you are about to tell me that most people will just need them to sell them on the GTN, why doesn't the same apply to columi loot? It's worth 10k at the vendor... somehow things change when you need gear? use the tools for which they are intended: if you need a drop, roll need.
Because need is a relative thing and without asking first the assumption is that you are trying to pull one over on the rest of the group.
I'm with Gabigool on this one. If you ask, I have no problem with you clicking need. But just click need and you go on my suspect list.