Many RV's with the short 4" vertical tube above the airbox do not have adequate heat available. Engines have quit because of the inadequate carb heat. I had a second heater muff around my exhaust stacks on my 6A.
I never use carb heat unless I suspect ice (has happened once in the air, twice on the ground........O-320, RV-6....twelve years, 1015 hrs). The Warrior I flew pre-RV used the same engine and induction system and carb heat was not part of routine ops in that plane. I do check it during every preflight runup, however.
There's no problem with using carb heat at any rpm. Like Sam I very rarely see any icing (O-320, RV-6A, 8 years, 400+ hours), and usually only on the ground (before the oil has warmed up) when the air is cool and damp. I have a 6" long heat muff on the exhaust cross tube, and a carb temp probe outputting on the engine monitor - carb temp warning has never flashed once the oil is warm.
