What CG said is right, but let me add on to it. First of all there is a saying that needs to be remembered. A nose heavy plane flies poorly, a tail heavy plane flies once! Now with that being said.... If you balance the plane with a full tank of fuel as the fuel is burned off the plane will become progressively more tail heavy, and the performance will worsen (and possibly be unflyable). On the other hand, if you balance the plane with an empty tank then at take off the plane will be a bit nose heavy (but still flyable) on take off, but as the fuel is burned off the plane will become balanced. In the end, if you run out of fuel you now have to dead stick a plane that is easy to handle because it's balanced. If the plane was tail heavy on a dead stick (because you balanced with a full tank) it's a pretty good bet that you won't get the plane back down in one piece.

In the end, a slightly nose heavy plane is a lot easier to deal with than a severly tail heavy one.

Ken