Burt, I thought we'd discussed this situation a few years ago and determined that we would go a 5 years or until salary maxed out in order to prevent someone from signing to 2 years and then renegotiating after initial year to avoid rookie bonuses. That's how I've always looked at rookie bonuses.
Here's the initial discussion on it:
http://predsff.com/forums/showthread.php?t=15276