Just to throw in my 2 cents. As a P-3 Flight Engineer my job was to compute the weight and balance (CG) of the plane for each and every flight. What I learned was when you move anything you change the CG. When we had changes at the last minute I preferred to put the added weight over the wings so the CG wouldn't change. The same goes for RC. If you move the fuel tank over the balance point you will still balance the plane at the same point. You may have to add weight to the nose because the weight of the tank has moved aft, but it will still balance the same.

As for the comments about balancing the plane with or without fuel. As the plane burns off the fuel the CG of the plane changes. If the fuel tank is forward the the CG will move aft. If the fuel tank is aft of the balance point the CG will move forward. If the fuel tank is directly over the balance point there should be no change in CG. So if your tank is located in the front of the plane balance with the fuel tank empty because this causes the most aft CG (which is the more dangerous limit to be over). If the tank is aft of the balance point, balance the plane with the fuel tank full. A full fuel tank will make the CG the most aft it will be in this condition.