That is an age old debate though. If the leader has specified that when you join the lfm and you agree then it's still your decision. If nobody has said anything, it's a PUG group then it doesn't relate. A party leader can't turn around after seeing you dropped something and say you have to give it up now without first specifying that they are running the raid purely to find a certain item for someone.
I do agree though that if you have agreed to hand over a specific item if you find, prior to starting the raid / quest, then you should honor your word. You were allowed to join the group based on you saying that you agree to the terms. If you didn't agree with them then you should never have joined.
If it is your guildies asking and you run with them and find it then you have decided to hand it over prior.
At the end of the day though, no matter what the situation is, the loot has your name on it and you decide to pass it over or not. Everything else that is situational is considered to be good manners or ettiquette. But then again if I run a PUG group I will expect players to pull everything for themselves. I don't expect to be offered a chance to roll. I myself always stick bound to character items up for roll (that I don't need) to whoever wants it and once I have put their name on it then they can decide what they do with it.
What I will never agree with though, is some player telling you that his character has more use for an item than you do so you should give it to them. That is a piker trying to take a chance with you and con you out of your fairly earned gear. If someone like that gets annoyed and squelches you I would consider that a gift. Unless I squelch them first