I concur with Mr.Betts,

The drill motors do have a bias, but as noted by Mr.Ross (post #7), here , The CIM's should preform the same forwards and backwards.
In my experience, there is always a little difference between individual motors, but that is easily corrected for by your driver. If there's a large discrepancy, try calibrating the drive system (use dashboard to set your joysticks neutral, or use the "cal" button on the victors; hold it in then go through the full range of movement for your joystick.).
In autonomous mode, a way my team has found to compensate for this drift is to use encoders on both of your transmissions. That way, you can tell your program (In advance, I'm not a programmer, but hypothetically, this is how I'd use the encoders) to increase voltage to each side until they are equal at x rotations instead of telling the motors to increase their voltage to y Volts.
What I'm trying to get at is that it's probably easier to just minimize the difference, but if you're looking to completely equal it out, you'll probably need some sort of feedback via sensors to your RC. Hope this helps!

Mike