The best policy, IMO, is what's been mentioned already. If hooked to city water, cut it off at the outside faucet if leaving the RV. If you're using your fresh water tank, cut the pump off when ever water is no longer needed.
As for the waste tanks, I always keep both tanks closed. I want full, or nearly full tanks, when ever I dump. Let both acculumate as much as possible. When my black is full, I make sure the gray is at least 2/3 or more full. I then dump the black tank, and then dump the gray. I then leave both closed until I need to dump again.