True, but DC motors are electically noisy, (voltage is DC, the current isn't) and you'll have a better chance of it not generating interference if you twist the supply wires. You MAY be OK if you don't have any small-signal (audio, etc.) wires running parallel (physically) to them. For noisy circuits like this, try to avoid using the airframe for a return path as much as possible.
A little OT tip: Even on "clean" circuits like incandescent lighting, running a return wire along with the supply wire helps to cancel the magnetic field (generated anytime current flows in a conductor), and will lessen the likelyhood of interference with your compass or magnetometer.