Mixture Control
It always seems a contentious issue this one and sort of goes agains the POH for most piston aircraft but certainly leaning slightly as you get to higher altitudes allows the engine to develop more power which is sometimes valuable. Conditions also need to be considered. I once took of from Samedan which is the highest commercial airport in Europe. I was in an arrow and on the day, the density altitude was 6700ft. Taking off in these conditions and climbing whilst fully rich would be a big mistake in mountanous terrain. I also find that my Mooney climbs much better with comprehensive engine management including leaning in the climb whilst carefully monitoring, temperatures especially turbine inlet temp and fuel flow. In most cases, if not confident then fully rich is the safer option but a good understanding of the particular engine installation and exterior conditions does pay dividends.