During my ab initio training (Up to GFPT) I was taught to leave the mixture full rich for all phases of the flight. This was also policy up to multi engine students because some international students in the past had leaned the mixtures a little too much during flight and ended up costing the flying school a lot more than their savings in "new engines".
During PPL training I was taught to lean the mixture during taxy, then the first thing on the run-up checklist is mixture full rich... Then lean the mixture again just after vacating the runway as part of the after landing checks.
Only during the Commercial and IFR navs are we allowed to lean the mixture during flight.