What he said!!!! You want the plane balanced so without fuel because if it's not then the plane will be tail heavy when it's out of fuel, or ready to land. There is an old saying about planes, "A nose heavy plane flies poorly, a tail heavy plane flies once". When you balance without fuel you are a bit nose heavy when you start your flight, but not enough to really make much of a difference. But when the tank is empty the plane will be back in balance.

Ken