Here's another refugee from Private Flying, I was taught the same as FNG, carb heat on before reducing power and off on finals. As I understand it, carb heat should be off before applying full power cos the engine does not need to be sucking hot air when it's being asked to produce full power (and heat).
I usually fly behind a Continental A-65 or an O-200 both notorious carb ice generators so I apply carb heat every 10 minutes or so in the cruise regardless of conditions. As the venturi effect and fuel evaporation process in the carb throat can produce a temerature drop approaching -50 C, outside air temperature can easily become irrelevant in a moderatly humid environment.
Check out the accident section in GASIL to see how many "engine stopped. Carb ice suspected" incidents there are. A lot of people are not getting it right it seems.