[Richard Wallace]

I will chime in here to support Al.

I always ask my team's students to measure free (wheels up) current with the drivetrain fully assembled, and compare that to the sum of the free currents of all the drive train motors. The difference is extraneous mechanical loss.

Extraneous mechanical losses are bad for your robot in several ways. As Al pointed out, driving those losses can overheat the motor. They rob power from your wheels, causing you to lose pushing matches. And they overheat gear meshes, bearings, fit points, etc. -- which accelerates wear and can cause early, possibly catastrophic failures.

Cantilevered loading of planetary gearboxes also increases extraneous mechanical losses. This particular loss is not easily measured with the wheels up. It is similar to the loss caused by excessive chain tension, and should be relieved as Adam and others have suggested; i.e., by supporting the output shaft on its outboard end, so that side load is not transmitted to the gear meshes.