BasicMicro

For this system you should put the encoders on the front wheels. In general it will work well. And yes, if one or the other encoder attached wheel looses contact with the ground.(or slips) you will loose accuracy. As long as they are on the ground and not slipping the speed/movement will be pretty accurate.

You will still have differences because of wheel diameter errors(eg no wheels are perfectly matched) and wider wheels will cause more slip/sliding since they may catch on the inner or outer edge during turns. For best results use very thin tires/wheels.