the idea about an empty fuel tank when establishing C/G, is that it is assumed a landing will be made with the tank empty (or nearly so) and that is when you want the plane to be the most stable and well balanced. the worst condition is first;...any amount tail heavy and secondly;.... excessively nose heavy. avoid these two conditions at all costs. even a plane that is a few/several ounces heavy from adding balancing weight will still fly decently with only marginal power compared to an unbalanced one.