If you start out from the get go with your tank positioned over the CG, then balance the plane accordingly, you will have no CG change as you burn off the fuel.

If you start out with your tank behind the firewall, you should balance with the tank empty, as this will be the condition when you are low on fuel when flying.

It is preferred to maintain a constant CG throughout the flight for obvious reasons. So a CG mounted tank is a good way to go. This was not as easy in the past as most glow engines were draw feed and not pumped. With the popularity of the gas engines, it is much more viable as the carbs are pumped, making tank location less critical.