I stand to be corrected but here's my theory.
Carb heat is only properly effective when the engine is hot. Therefore it is not as effective to apply the heat once the power has been reduced as the amount of heat given off to combat ice is less than if cruise power has been applied for several minutes before its introduction.
There are some days when icing is very prevalent, there are others when it is not.
The facts are clear and proven though. If carb ice HAS built up it takes a lot longer than most people think to clear it. Up to 40 seconds in fact. I don't believe that one would wish to find carb icing on base leg and have to wait 40 seconds for it to clear whilst in the latter stages of the final approach.
Basically, I perform the check downwind and if there is signs of icing, redo the check just before pulling power back on base.
If the revs did not rise after applying downwind then I consider it safe to slide from base to finals without another application.
I remember learning to fly 14 years ago and performing the carb ice check "just because I thought it was right to do so". Since then I have found myself in numerous situations where applying the carb heat has given a 300 rpm rise in power with some serious coughing and spluttering of the engine to boot.
As far as I'm concerned, keep applying it even if it's habitual or seems unecessary at the time.
