I can't vote because we just got our first rechargeable. I've been pondering this very question though. Is it better to leave it go dead completely before charging, or keep topping off the charge?
Some batteries develop a memory if you don't let them die periodically. You end up losing part of the battery's life because you haven't used it. I hate the thought of picking up the toy and the battery is dead, but I also hate the idea of losing run time.