Hi Swathdiver
Running-in electric motors is not strictly necessary due to the progressive brushes adaptation with use. In electric flight we do it in order to obtain max. performances from our motors from the very first moment, it's imperative....fortunately for us, brushless motors and LiPo batteries have made e-flight much more easy.
In our tanks is more probable that we burn our boards before our motors break.... It was just a suggestion to enhance performances, but repeat, not stricitly necessary. When runnning static under heavy loads as in your gearboxes running-in process, the sparks between brushes and collector tend to be stronger than those that appear in well runned brushes. These sparks can slightly damage the motor's collector, but nothing really harmful, don't be worried , that's the reason low voltages and no loads are recommended for breaking-in of e-motors (excuse me if I merge the terms running-in and breaking-in, but I'm unsure about which is the correct one)
Cheers
Jose