In my opinion "Radar heading" is only good use of phraseology when used by pilots when they check on to a new frequency intending to tell the new controller the aircraft has been assigned this heading by the last controller. That is to aviod the new controller to believe that the aircraft is still inbound some fix and just reporting his present heading for information.
I think "Radar heading" should not be used by controllers. I think better phraseology for "make present heading a radar heading" would be just "continue present heading".
Just my opinion
C172pilot