It's not like you're balancing it on the head of a pin, you have a range of CG to work with. Putting the tank on the CG is ideal, but not possible in many aircraft. What's more important IMO is having the fuel tank height correct for reliable engine runs. The centerline of the tank should be at the carb centerline in a level attitude.
The manufacturer has done all the work, so put the plane together and enjoy. Unless you change something, the CG will probably be very close to what's recommended when you're done building. Leave the battery and servo install til last, then you can move them around to fine-tune the CG.