Wasn't taught much about it in the private days, but was teaching it heavily as an instructor. Got tired of getting into an airplane , doing a run up and having the plugs foul. I lean on taxi, sometimes climb (depends on a/c and what altitude I'm going to), and always when I'm pulling 75% power or less, even down low.
When I was flying out of mountain strips in Idaho, Montana and the like you have to lead on take off to get full power. I have never trusted the lead of peak thing. I know some guys in Bonanza's that swear buy it, but it's not for me. The airplanes I usually flew it was a luxury to having a working EGT gage. My usual trick was lean until the engines changes tone, that would be about peak EGT then 3 half turns in. (each half turn is about 25 degrees, so you would be running about 50 -75 rich) Worked about right for most trainers I've flown in. The engine analyers mentioned are sweet but owners get paranoid about 10 degree's of difference in a cylinder. They flew the pants of the thing before with an old standard EGT now they have this toy and try to lean it for the sake of 2$ in fuel. Well the engine is about 30,000$ for the Beech so I'll just stick to a little on the rich side is O.K. for me. I'm not saying the gages aren't useful, just takes some skill to do it right. (which I don't think average joe pilot has, that's not flying the thing all the time) just my .02 cents