My own GA experience while training had Carb heat on for the downwind checks then select cold when complete, while turning base carb heat on again before throttle idle or near idle revs and continue to fly approach with carb heat on until on final down to 300ft where carb heat is selected cold, I once questionned this in the context of a go-around situation to which my then instructor said, "its the way we have been told to teach it and besides if a go-around is required you have time to select cold and commence go-around from 300 ft for full power"
Well, I agree with what he said, but didnt feel particularly confident with the "its the way we have been told to teach it"..........got to love those CFI's on a power trip.