Most RC engines are ABC or ABN engines which mean they have a tapered cylinder bore which allows continuous mating at the top of the piston/cylinder area which experiences differential expansion due to higher temps.

If you run your engine at an idle speed which does not heat the cylinder hot enough to expand, you risk unnecessary wear of the taper-it is what gives your engine compression.

ABC/ABN engines were originally designed to be run at full throttle in racing.

I personally would not idle my engine excessively. I would never idle my engine to break it in. To do so is contradictory to the theory behind our ABC/ABN engines.