In TOC (which I run almost every day), a DK dps (in blues and greens and maybe 3 purples) rolled on a tanking ring against the Paladin tank, who stopped just short of throwing a fit. When the DK wouldn't hand over the ring and I tried to whisper the Paladin into a better frame of mind, the Paladin left the group.
In the randoms I've run, I've supported the policy that if you can and will use something, you can roll on it. Tanking classes can roll against the group's tank if they're building a tanking set. My underlying philosophy is that people need to gear up to perform their rolls and everyone has the right to gear up.
Now, is it frustrating to lose a roll on something you've been farming forever?
Can you whisper others to make an agreement on a specific item or to request that they let you have the item after they win it?
Can you demand that others pass on an item for you or give it to you or get in a huff if they won't?
You can ask nicely, but you can't insist. You can give people the opportunity to be generous, you can lay your plight at their feet and request that they take pity on you, but you have no right to demand what they've won just because you've failed at it longer or you're the "right" spec for it.
It comes down to this: basic manners dictate that even though you may be gnashing your teeth with frustration over a lost roll, you don't get to take that frustration out on your group. Lose gracefully or don't do randoms.