There have been a few accidents where pilots have claimed to put carb heat back to cold on short final to find the engine stops over the runway or goaround.
I,ve always taught in 150/152 to keep carb heat on until rolling on touch and go. Always treat the 152 as if it is a 150 just in case it is flown one day.
It takes no more effort to apply carb heat cold than full power.
I have hardly ever had carb icing in PA28, but perhaps because its always checked every 5-10 mins.
Good time to demo carb icing is on an early morning start on the PA28. After 1 min put carb heat hot, rpm falls then after few seconds rises. Carb cold and the rpm has risen 100-200 rpm.
could be just the effect of warming the carb or damp in the inlet tract being sucked into carb producing ice in cold carb.