i bolt my engines on my plane and have at it, no breakin on the bench. what i do is keep the low end very rich and the high end rich, but lean enough where it will clear out.
then i fly it just above the point where the engine doesnt load up for a min or so (usually a little above half throttle) , then lower the throttle and let it cool off/load up, then i advance to the point where it clears out and alternate till its time to land. i do that for a few tanks with a small prop for 2 strokes (11x7 for a .61),
so far i havnt had any problem doing that, and its more fun than watching a engine spin on a bench (which is still fun )
just keep the plane close incase of a flame out, which if you listen to the engine, it is really easy to tell when the engine is loading up/dieing.
if you want to break it in on the bench, use the above method only keep it on the ground.