I always used to teach leaning as part of Effects of Controls 2. Why? It was in the syllabus I was tought to use! But then I pointed to point out that Lycoming themselves mention that leaning below 5000' should not be necessary. Only ever lean in the climb to prevent rough running and in the cruise - with care.
Why? It's because to break down excess fuel into it's basic elements takes a lot of heat and this aids cooling. Excessive leaning leads to a hot engine and potential problems as a consequence. On one group I was a member of, one pilot used to lean a lot - probably because he did the groups books and wanted to save a bit of money. After he went on a week long trip where he had obviously been leaning a lot, we needed to change the engine. Not much of an economy there!
For effective leaning, A Cylinder Head Temp guage or even better, an Exhaust Gas Temp guage are required. Without these, leaning is guesswork so it's better to err on the safe side.