I have never "broken in" a motor.
I have never heard of "breaking in" a dc motor for any other application either.

Is this something one does with larger scale RC stuff too?

It seems to me that running the motor with no load just creates more friction and wear faster, reducing the overall life of the motor.
(This might be a good thing to encourage if you're selling motors!)

Wouldn't this wear happen with normal use, and actually give you some running time for your time and trouble?

I've read alot of posts about breaking in motors, but I'm skeptical.