If all you have is a slight pull to one side, I'd keep it. It will probably loosen up in the bushings, or gears will mesh better over time. Since these tanks dont move fast it takes a while. It could also be the motors, even within the same batch of motors, all will run differently. but again as the brushes get seated in and bushings in the motor break in they should get pretty close to the same speed. If it really bothers you, you could take the gear boxes out and check the gears ect, lube stuff up and clean out the motors with motor spray. But its just eaier to bump the stick every so often to keep it straight.