Fuel tanks are mounted against the firewalls because engines often can't draw from farther away dependably. We also rely on muffler pressure to help make the fuel flow even more dependable for tanks mounted close. Some engines draw better than others, but model designers have no clue which you will use, so the recommendation is always to mount it as close as possible.

Everyone also works out CG location with an empty tank because it works well for everyone to do so. Think of the confusion and extra effort that would be caused if we didn't. You would have to use a certain size tank with a required amount of fuel, or have specific requirements for load and placement. If you didn't follow the recommended load, you'd then need to compensate for what you did differently. As it is, most modelers can't work out where the CG should be when there is a simple no-fuel standard.

Find and adjust your CG with no fuel. Look at all the models flying next time you're at the model flying field. Most of them were setup that way, at least the ones the modeler actually checked and adjusted the CGs, anyway. It works great, doesn't it.