Some opinions:
The reason for heating up the piston/cylinder ASAP is to lessen the wear on the bushings (conrod) during the first couple of minutes of running.
Picture the lower end of the conrod riding on a film of oil that keeps it centered. The faster it spins, the more force to keep it centered on the crank pin.
Now add side forces imposed by the piston going up and down. This will cause the rod to not be centered on the crank pin but to rub against it causing a bit of polishing for a final fit.
Now picture extreme loads because the piston/cylinder are extremely tight. This can cause excessive wear on the conrod so that the bushing is enlarged excessively.
This also applies to the piston/cylinder. Heated up properly, it wears more slowly to polish the piston/cylinder fit and allows particles to be carried away. Run slowly, the fit is very tight and more grinding than polishing occurrs.
The best way to break-in an ABC(ABN, etc.) is to quickly get it up to operating temp (rich side of peak) using a smaller than normal prop. After about five minutes, the parts are polished and the engine is initially broken-in. You will probably see an increase in performance for about an hour, then it should be fully broken in.
Is this the only way of breaking in an engine? Of course not. As people state above, they have gotten good results with various methods. This method should, however, make your engine be all that it can be.

George