The deciding whether to bet or not seems unfair to the team that has a better record. As the playoffs go on, a team that slips in to the next round will just rest on what he has, and deprive a truly dominant team a chance at decent winnings.
I can see banking a certain amount, but ultimately, I would think every round should be a "double or nothing" type situation where the winner takes the stack - or at least a majority of it.