Great advice, you guys.
I've only ever leaned in the cruise - can't imagine why anyone wouldn't do that, but the leaning on the ground bit sounds like something everyone should know about and at least consider.
The Cessna 172 Pilot's Guide I've got (by Jeremy Pratt) calls for:
Full rich for takeoff & climb except when departing an airfield > 3000 AMSL, where it calls for leaning for max power;
Leaning in the cruise to 25-50 rpm less than peak rpm on the lean side for best economy;
Use of full rich with more than 75% power set, noting that above about 5000 ft, full throttle will be less than 75% so leaning would be OK.
Doesn't talk about leaning on the ground, but it seems like a bloody good idea to avoid plug fouling.