i also think when they outgrow a hbb by height. watch the videos out there - it's their noggin' and spine that you're protecting with a hbb.
decisions like this always need to be made on the basis of your child's size, physical development, and maturity, not on the minimum numbers used by carseat manufacturers and state gov'ts.
also, if your car has side air bags and they fall asleep leaning on the door frame (or just sit that way), that could be bad in an accident.
i am a cautious mama by nature, though - and we have bad winter driving here - getting crunched in the side of the car by a driver who couldn't stop in time at an intersection is a real risk here.