If you get into the nitty gritty, I believe there is a small difference in the load and current draw for each direction, but I have never had a problem and therefore never worried about researching why. There may also be a slight difference in each motor as well, even if they are the same model. The main issue you might have is getting the robot to track straight over a long distance. You can calibrate this with the code or, if you need to adjust it on the fly,via a trimpot on either the motors or for higher currents, run a trimpot into an analog input and use it as a reference to bias one motor or the other.