Faultygoods -
Are you suggesting that if the OAT is above -5 degrees then carb heat should never be applied. If so you seem to know a lot more about the subject than the CAA amongst other authorities.
I may be wrong with the next bit, but In a dim recess of my brain lurks a piece of information that for (UK) certification purposes the carb heat must be sufficient that when max heat is applied there will always be a rise in temperature great enough to ensure that the temperature in the venturi is above freezing. (this is as opposed to a warm air system which I seem to recall being fitted to some British engines and could cause the problem you mention at temperatures close to freezing).
Hence unless you have carb heat guages the rule is either all or nothing, don't try and guess what is happening temperature wise.
Incidentally - why does it now seem to be the norm to turn off the carb heat at 300 feet prior to landing, in case of a go around. I was taught (and have taught for the last 17 years) that it should stay on to the ground (this is in Cessnas where it can easily be selected off with your thumb as you open the throttle), but I was pulled up for this on my last Instructor rating renewal.
Surely there is just as much danger of carb ice forming in the last 300 feet of an approach at a low power setting.