Charging a battery produces heat in the battery. Heat is a battery's enemy. Therefore, leaving your Sprint HTC Hero charging for extended periods of time when it is already fully charged should be minimized. I try to take my Sprint HTC Hero off of the charger as soon as I notice it is finished charging or perhaps a little after.
Sometimes my HTC Battery widget will jump right down to 99% after I take it off the charger, other times it will remain at 100% for a while. It might just be that small extra time I leave it on the charger that lets it stay at 100% longer.