I wish the umpires wouldn't get to decide if it is a balk or not. If it is a balk, it should be called as such on a regular basis. It's not fair for the kid to do that all year, not get called, and then get called for it in a critical game. If the umpires had been calling the rule all year, he would have been out of the habit of it by the time the playoffs rolled around.
We had a kid get called for adjusting his hat while in the stretch position last year. I obviously questioned his called, but when I looked back at the rule book, essentially anything you do while in contact with the rubber that is not your regular pitching motion is a balk. The umpire was correct in this case, but it something that not all umpires enforce, so the kids don't know when it will be enforced.