When you go to the time, effort and expense to rebuild your carbs, they will have absolutely pristine innards. It would be a crime to connect them to a contaminated fuel tank. Now’s the time to evaluate the condition of your fuel tank. At a minimum, drain or siphon all the old fuel out. The drain plug is located near the rear master cylinder. Obviously, do this outdoors to avoid fire hazard.
Once the fuel is out, you can see the inside through the filler next with a strong flashlight (somewhat easier in a dark garage, so you may want to push the bike back indoors for this step). I usually remove the fuel sending unit for a better view inside the tank…but, this means you will need a new gasket for the sending unit. Note: the only sealant I’ve found that will prevent the threads of the drain plug from leaking (even with the OEM crush washer) is Permatex Aviation Form-A-Gasket No. 3 Sealant.