Follow the recommendations of the POH-AFM of course. But the idea with carb heat would be to apply it well before a reduction of power leads to loss of the engine due to ice in the carb throat. You might not know there is carb ice until the power reduction, so it is a good idea to apply the carb heat well ahead of final approach. Waiting until you are already on final may mean losing the engine at a bad time. Because carb heat takes away quite a bit of power on most engines I prefer to do a quick carb-icing test while I am still at a good altitude (1,000 or so) and just leave it off unless of course I detect carb ice. That's just me and I do not know anyone else who does that.