I think people are getting too hung up on this, where motors are concerned there will always be noise. The best thing to do is just wire them up and see if you have any interference? this can be dealt with by grounding shields, toroids or better quality cable. I use a single shielded cable and have no problems with it not being grounded. Most cases it is just to be aware that you MAY suffer and not that you will have? Ground loops can cause more issues than a bit of noise after all we are not in the days of valves and tubes anymore. Always look at routing cables away from the motors as best you can and keep it tidy. Just enjoy your build rather than frightening yourself over something that may NOT happen :-)