I know that slipping the wheel when sprinting will put the tire against the left chainstay; I'd expect that if you're slipping it by backpedaling, it's rubbing on the right (driveside) chainstay. My guess is that you're using a freewheel hub with a QR skewer (which is what I ride). This should be OK as long as you have the skewer tight enough and aren't shy about using the brake.