I've looked in my three "Prouty" helicopter aerodynamics manuals.
He does mention C of G a few times, but nowhere does he answer the OP's question directly. My own take is that it is probably more about the aerodynamics of the fuselage as presented to the oncoming airflow, i.e. mainly parasite drag related and the least "draggy" aircraft nose up/down attitude will vary from helicopter type to type.