Keep in mind the initial location for a new model is intended to produce an aircraft that is safe to fly. Good designers match that CG up with the control settings to give an easy to fly model for the average modeler. The vast majority of models we encounter will be provided with Balance Points, control layouts, and surface throws to do just that. Since the early days, the accepted basis for balancing the average model has been to do so with an empty tank. Designers usually are smart enough to include clear instructions when their design would require other than an empty tank for balancing.

If you're in a quandary about the full/empty question, and there are no instructions, show your plane (or a picture of it) at a LHS and explain your worries.

The EMPTY TANK convention is so well established in our hobby that not one of the various CG (balance point) location applications developed over the years (many of which are available online) mention tank location or weight. They require you to measure the model and input the measurements and hit the Enter button. They would certainly state things they thought were important to include. They leave out "what everyone does already" and that is to ignore the fuel for the "usual" model.

Want to see what matters for balance point? Check out ghttp://adamone.rchomepage.com/cg_calc.htm It is one of the most "complex" CG locator. It requires you measure the model.