FCC,
One more opinion:
A two cycle ABC (ABN, etc.) engine is designed with a tapered cylinder. When you start it, you want to get it up to operating temperature as soon as possible. This causes the cylinder to open up at the top and lessen the tightness/friction.
The first runs should have the engine running in a fast two cycle, but not full lean. This causes the cylinder to expand and allow the connecting rod and other bearings to have a smaller load, which is much better for the rod bearings. Light loading is the same reason you should use a smaller prop.
After the first few short (minute or two) runs with complete cool down between runs, the bearings are better able to handle the stress of rich runs.
Remember that tapered cylinder? If you run-in the engine rich, the cylinder will cause excessive friction because of the tight fit and will eventually wear a fit at that temperature. When you lean it out you will then cause the cylinder to expand and the fit may be too loose to have a good seal which means you have worn out your engine.
If you are flying a sport engine, this may not affect you as much as if flying a competition engine.
Remember, this is just an opinion, your mileage may vary