I'm of the opinion that if you pay to go to the game then clearly you have the right to boo, but personally I feel it's crass and in most cases pretty unnecessary.
Unfortunately however, the growing principle that we should openly boo and regularly boo is a testament to how our fan base continues to change for the worse.
We've got into a cycle where some will moan at the players without considering our own influences and how we as fans can impact the game, and vice versa.
Whereas really both parties need to consider how they can influence each other: the players need to attract us but we also need to give them a boost on the pitch.