Problem with trying to lean the mixture correctly is usually a lack of information displayed in the cockpit.
An EGT gauge is very usefull.
However in older trainers they're either not installed or placarded inoperative. If working it's usually only hooked up to one exhaust stack and it may not be the hottest one.
It could have been installed on a random cylinder rather then the leanest one.
Yes, surprise, with carbureted engines they rarely run equal.
This has to do with intake plenum variations, carburetor design and so on.
An aftermarket engine monitor gadget should show you all cylinders.
Leaning for ground ops is a little easier.
For the usual 160-180 hp 4 cylinder : : after engine start run up to 1200-1300 rpm and start leaning.
Watch for initial rpm rise then a drop and a stumble. Lean for the rise.
You now have the correct setting for the max power setting you should need for taxiing. Unless you're operating off a potato field that is.
Also when going full power ( in the runup area!) the engine should stumble and die before reaching full power.
Problem with just arbitrarily pulling it back a little in the taxi is when you forget about it you may overheat a cylinder in the climb as the lean is not aggressive enough to prevent full power.
In the USA you're always taught to lean on the ground also.