I wouldn't say booing is NEVER acceptable, but it's never acceptable because of how the team is playing. If a player does something REALLY stupid like a flagrant foul that is solely for the purpose of hurting another player, for example. Boo his ASS.
Another example where it might be acceptable is disrespecting the coach. DMAC did it blatantly one time by giving old man Weber a hard shoulder while going to the bench. Boo his ASS!