Moonburn,
" On the PA28 carb heat should be checked on the downwind and then selected off for the approach as selection to hot for the approach can lead to icing on this aircraft" I cannot identify the source
I got curious and just checked, the source is the PA28 Pilots Operating Handbook. The guidance is followed by an explanation that the approach should be flown with it off in case of a go-around and engine detonation, not in case of increased risk of icing as previously postulated.
Can I add this
http://www.caa.co.uk/docs/33/ga_srg_07webSSL14.pdf as another more authorative description of carb icing in general.