I guess it's more a situational thing for me. For example, if the QB has an awful game or a receiver drops a ball I don't boo (they're just kids after all). But, if it's clearly a coaching issue I think booing is fine. For example, if the punt team comes in on a 4th down near mild field with 3 min or less in a game where they're down by 7 or less...I think booing is fine. Same with a draw play on 3rd and 20 late in a tightly contested game. In these instances, I have no problem with boos, as you are showing displeasure with situations where the coaches are essentially waiving the white flag.