I wrote out my rules for booing many years ago. They're for me, not for everyone:
-- Cheating
-- Intent to injure
-- Unacceptable behavior (say, a player flipping off the fans)
-- Egregious delays of game
-- Lack of effort (e.g. not running out a ground ball)
-- When it would be really funny
That said, we salute the intensity of the fans who have made the atmosphere at the Bank so special. It's hard to just turn it off after an emotional game and say "Get 'em next time." There are always going to be bozos who do unacceptable things like taking out their frustrations on someone's family, and there's no excuse fort them. But booing in the moment is barely a misdemeanor.