I was taught to prefix the word "Helicopter" to my Aircraft registration and i traditionally do this when I'm flying rotary. I have never been pulled up over it when talking to ATC and regardless of whether or not it is standard or acceptable phraseology (by CAP standards) I personally think it immediately allows ATC to identify the possible capabilities of the aircraft type (not the pilot!) quickly.
Once I've initiated contact I usually abbreviate to Helicopter XX afterwards.
So there!