I have a lower geared version of that motor. There is some backlash – perhaps as much as thirty arc minutes? Probably not more – this is by feel only; I have not measured it.
However, the backlash is an absolute quantity. You won’t get “more” imprecision as you keep running the motor, as long as you can keep up with the counter.
A high resolution encoder on the output may be able to tell you where the axle is even as it moves because of backlash, but that won’t help you very much, because the motor has no control over that small amount of movement. For most intents and purposes, a very high-resolution encoder on the output shaft will simply be equivalent to the encoder on the input shaft. And if you think about it: at 131:1 and 64 ppr, you’d need a 8192 ppr encoder on the output shaft to get the same level of precision. That’s… very high