I agree that stop collars can produce more drag, but they are not the only option. Attached is a photo of my Jaguar lightweight that I built four years ago. I am doing some work on it. I originally had two stop collars on it. I put a .005 Slot Car Corner axle spacer, then a .100 axle spacer on the outside of the motor pod, opposite the crown teeth. This limits the axle side play away from a tight gear mesh. The original axle collar is still located behind the crown gear, but it no longer makes contact; the .100 spacer is the limiter. I will probably be removing the collar. Although not totally necessary, I will probably put a spacer on the other end of the axle that will limit the axle play toward tight gear mesh. That way, instead of side load to the crown gear, the side loads will be to the two axle spacers. Once adjusted there will be no more drag than axle spacers. I will see how it works when I have a chance to test it.
Russell