Bell 204/205's have a green arc on the Nr/N2 gage that "allows' you to operate as low as 97%Nr for cruise. The UH-1H manual allows cruise at 97%, in fact all of the cruise charts are set up for 97%Nr (6400rpm, N2)cruise. In the UH-1H there is a chart that allows for a correction to 100% Nr. These birds were certified under the old Civil (CAA) rules which have been changed numerous times under the FAR system. The original reason for the 97% Nr cruise was for extending range, which amounted to about 4% advantage.
I had the experience of certifying a UH-1H under the newer FAR's and found a major problem with the aircraft response to sudden engine failure. I was at 97% Nr, 4000 ft Hd, 108KIAS(Vne), and 9400lbs.(9500 is maxGW). I chopped the throttle and was supposed to wait 1 second before reducing collective -- needless to say I didn't wait. The Nr dropped through the floor and I had to use the engine to keep it from getting below recovery Nr. During the recovery I bumped the mast, though slightly, and damaged the rotor mast dust boot.
Later, I was able to uncover two US Army test reports and one US Air Force test report that identified the same problem. Needless to say we certified our aircraft at 100%(6600rpm N2) and no lower. In fact with our installation of the PT6 there is no beeper switch for Nr.
I would never recommend flying a single engine helicopter in cruise at a lower permissible rotor rpm than its stated 100% because of the above. If you are in a high Hd, high GW, high speed condition and the engine quits, the recovery will be a monster and may not be possible. If you have to fly in that condition, "spring load" yourself for an engine failure.