John,

Specifically, you should not be leaning if your engine in making 75%
power or better.

Not necessarily so. It is perfectly okay to lean at 75% power, the
manual for your airplane has a fuel burn for 75% with lean mixture.
In fact, if you don't lean when operating at 75% with that Lycoming,
you will significantly reduce your range and endurance and possibly
foul the plugs. www.gami.com has a good discussion of leaning and
what's going on during combustion. It may be perfectly okay to lean
at power settings above 75%.

While cliches may have some basis in fact, few of the OWTs do. Some
are erroneous carryovers from radial engine days (such as a power
reduction after takeoff on most horizontally opposed engines) and
some, such as not leaning below some arbitrary altitude come from a
very simple misreading of the Owner's Manuals/POHs where the manual
said to lean in the climb above some altitude and some folks
misinterpreted that as meaning not to lean in level flight below that
altitude and didn't notice that the cruise performance charts for
those altitudes stated clearly "with lean mixture".

All the best,
Rick