Filling fresh water tank before trip, good or bad?

We know our trailer weight, and. A full tank would add 600 lbs. which in turn would cause us to be over GVWR of the trailer. We travel with about 1/4 tank for flushing, hand wash, minimal needs. Only one time did we ever find a Nat'l Park with no fresh water source, and then, the camp host let us fill our tank from his hook up; so we have always found water. Additionally, any extra lbs does affect mileage whether you feel it or not.