A forward balance puts the fulcrum at or above your hand. Putting the fulcrum behind would just feel different and reduce the control, as well as causing the tip to want to rise with an open bridge as Bill already said.
With the balance behind your shooting hand, it is like choking way up on a baseball bat or golf club. You just might not have as much power available and control. Also, the way the muscles have to flex during the stroke could be slightly altered. Some muscles have different strengths depending on which way they flex. Take the jaw muscle. You can exert great pressure biting down on something, but if your jaw was taped shut, you can't exert the same amount of pressure opening your jaws. While that may be exaggerated compared to stroking a pool cue, weight behind the shooting hand will simply pull slightly different than weight at or above the shooting hand.
It is a personal preference ofcourse.
Kelly