Good question MLoud. Knowing what to do and doing it right can potentially save you a lot of money. As far as draining it it's up to you, it won't cost you any extra but it's whether or not you want to keep it clean for the next person to move in to your home.
A. Keep it the way it is, no harm done
B. Clean it for the next person who moves in
Any other water damage related concerns can all be answers from http://www.waterdamagelocal.com/water-damage-survi...