You can't "over-charge" so feel free to charge it all you want, whenever you want... but keep in mind that rechargeable batteries can only withstand so many charging cycles. So, if you charge it unnecessarily or charge it every night, the battery is more likely to die prematurely... but whatever. Batteries are too inexpensive to worry about how you treat them
FYI- A charging cycle is not everytime you plug it in. For example, if your battery is at 75% and you plug it in, you only use 25% of a charging cycle. Therefore, it doesn't matter if you let it die down more or just keep it topped off.
