One side moving slower than the other can be a from a whole host of reasons. First, make sure your trims are centered. Then the same amount of track links side to side (or sag). Are both gearboxes evenly greased?
Same method, but start from the tracks up, looking for drag, etc, until you get to the motors. They can be uneven too.
I wouldn't worry too much if there's just some drift to one side while going forward (trim or just steering will fix that), but it there's a severe tracking problem or one side is certainly under strain opposed to the other, you may want to look into it.