You should always use carb heat when the engine is still making power, prior to entering the pattern. When the engine is throttled back it may not produce enough heat in the exhaust muff to be effective if you have an accumulation of carb ice.
If you are suspicious of carb ice you can "clear" the engine, that is add some power to generate a little heat. Back the bad old days of Cubs, which were real ice makers, we were expected to perform power off landings, (to a spot) and when things weren't going right we would "clear" the engine. The instructor or examiner would sometimes let you get away with it, often with a caustic comment about how if the engine were to quit you should be able to make the runway anyway.
George
