Ok, IMO this is a delicate issue. On one hand, yeah, you should be able to roll on whatever you want to roll on. On the other hand, the need/greed system was built for a reason. Common sense dictates that if you want an item not built for your class because you want to sell/replace the modifications, you do not NEED the item, so you SHOULD do a greed roll. Same thing if you want it for your companion, that's really a greed roll, not need. People who roll need on items they DON'T need make the game a lot less fun because they take items that other members of their group actually DO need. Personally, I don't think there should be a rule against it, but any way you look at it, because of the way the system is built and designed to give an advantage to people who need the item over those who just want it, it is very rude to click need when you someone else in your group needs it more. So, If you're running a group with 3 Sith and an agent, and come across bounty hunter stuff, I say it's free game, but if in that same situation, it's 2 Sith, a bounty hunter and an agent, please let the bounty hunter have the first crack. That's just etiquette.
Increased stats for your PC=Need
Increased stats for Companion=Greed
Stripping mods for sale=Greed
Replacing mods because you think the item looks cooler than what you have=Definitely Greed
You see what the difference is? The system was built that way for a reason. So, should it be against the rules to click Need on something you don't need or cross class items? No. But it is rude, and I don't see why anyone should have to put up with it, if they don't want to. IMO it is a perfectly valid reason to boot a PC from a group, and refuse to group with them for future missions.