[Chris is me]

Assuming you're talking about drift over driving straight for a long period of time, and not "drift" in the form of the robot jerking slightly in one direction as it starts to accelerate. The latter is a direct result of varying chain tension in a way. If you've ever seen powerhouse teams set up their robot in autonomous, you'll sometimes see them push the robot forward and backward a few times as they are lining up their autonomous routines. I believe part of the reason for this is to ensure that both sides of the robot drive base are on the leading "side" of the chain backlash before the match starts, in order to try and eliminate any immediate jerking in one direction at the beginning of autonomous mode.

Drift can occur for a variety of reasons. If your speed controllers are not calibrated correctly, you may actually be outputting different signals to both controllers at full forward / reverse, which would also cause drift. I know back in the days of the Victor 884 this was a leading cause of this drift. Varying mechanical inefficiency is one of them, and chain tension can be a factor there, but if the robot is continuously running in one direction and the chain isn't over-tensioned it probably won't play a HUGE part. Differences in gearbox machining / assembly, bearing efficiency, chassis rigidity (this is very important), CG distribution, etc. can all play a role.

All in all, if your chassis drifts about a robot length in one direction across the length of a field, without corrective software, you're doing very well.