If your current instructor says that [leaning is] really not worth borthering about (below 3000' or otherwise), get a more experienced instructor.
or one who can do arithmetic!
For a 200 hp Lycoming IO-360 run at 65% power, the difference between the best-power and best-economy settings is about 1.5 USG/hr or 6 litres/hr. Over the useful life of an engine, that's going to add up to something like 10,000 litres of AvGas. Depending on your outlook on life, multiply by either pounds per litre or trees per litre to understand the impact. Note that full rich mixture is not 'best power'. It involves burning even more fuel for less speed.
But please let's not ignore BEagle's point. Running continuously high CHTs is not recommended. Leaning does not necessarily increase the CHT, but it can do at approximately best power. The kindest way of running for the engine is properly leaned but at a medium power setting -- say 65% rather than the maximum permitted 75%. That will do more for the CHTs than cooling with buckets of excess fuel.