Somewhat contrary to Dar's thoughts here's an actual experiment I did to see what would happen if a new ABC was run extremely rich.
http://www.rcuniverse.com/forum/m_28..._1/key_/tm.htm

What I learned from that was the best setting to start running them in was at a point right on or barely above the break from a 4 stroke to a 2 stroke. This is usually somewhere around 2000 below peak revs. I start the first tank at this break point and the revs increase slightly as the tank empties. At the beginning of the next tank I reset the needle so the starting revs match what it was at the end of the previous tank. And so on for several more tanks (about 5 minute runs each). This gradually increases the temp of the engine and allows the parts to adapt slowly to the increasing temperatures.

After they're run in I also run them quite rich for flying. Most of mine are used in CL stunt so they spend their life in a continuous 4 stroke but my RC engines are also kept quite rich.