I played a game recently where I purposely did not promote a Pawn for good reason. I've done it in a few other games, but it didn't make any difference. I've also had a couple of opponents not realize they could promote it on the eighth or hold off for a later move of the Pawn. In fact I believe that game is still on going. It's been a back and forth kind of game. He had to take the not promoted Pawn with his Cardinal. Had I promoted it, it would not have gone as well for me.