I am not saying that one will definitely have problems if they leave their Sprint HTC Hero, or any device really, plugged in for extended periods of time. I am just stating my experiences and how I handle most of the consumer electronics that I plug in to recharge.
As I said above, and I have tested this with a friend's smartphone too, if you leave your handset connected to the charger, even after it is finished charging, there will still be a slight bit of energy going into the device to maintain that 100% charge. Thus, the battery will remain a bit warmer than if you were to unplug it immediately. I am not saying this will dramatically reduce your battery's overall lifespan. I am just reiterating that anything that I can do to reduce the amount of heat that my handset's battery is exposed to, I will do.