In my humble opinion, operators of water cooled normally aspirated (non-turbo) engines don't need to worry about going too lean if they pay careful attention to listening for the sounds of detonation (especially at low rpms and high loads).
The knock sensor (if you have one) will help to make sure you aren't doing any damage. Otherwise, the lean burn Honda Civic VX guys would always be complaining about holes/cracks in their pistons and burned valves.
Once you get really lean, power decreases, combustion temps decrease and chance of misfire increases.
Below are a few graphics from our discussion a few years ago:
http://www.gassavers.org/showthread.php?t=8719
With changes in altitude, pilots of air cooled aircraft engines have a lot more need for mixture control than we do so it is of special concern to them: