Some run the motor low speeds in each direction for about 30-45 minutes to evenly wear in the brushes. That definitely makes sense to me.

Some also do that with gearboxes, even using lapping compound (for automotive valves), then cleaning and greasing. An equal amount of guys are dead against the compound unless you're able to dismantle and absolutely clean out every bit of the stuff.

I've done the motor break in myself and usually do if I think of it, but in terms of the gearboxes, I'm not worried about it. I only use Tamiya gearboxes at this point and never had an issues with them. If I was working with a $400 box, heck yeah, I'd spend a few hours breaking it in. If I was dealing with a cheap HL, maybe I'd use lapping compound to mesh the gears better. As it is, I've never had even the slightest issue with Tamiya gears (2 sets are my most used, still running after over a decade), except for a broken bearing and a bent shaft, both from impact from height.