Xnr,
You have to watch those FSI instructors, they tend to color the stories with semi-half baked warstories that have only a glimmer of truth.
The S-76A uses 96% Nr because it allows a better climb rate at fixed horsepower (that is when the engine is temperature limited and you are not on the torque limits of the transmission). The rotor is more power efficient at low speeds when it is at low Nr.
Almost every helicopter is that way, it has to do with the fact that the rotor is designed to allow high speed flight without stalling. The blades are wider that needed to hover, so they need somewhat more power than if they have less chord. One way to make the rotor behave like it has less chord is to slow the rotor down. This drives the collective to a higher angle (for the same lift) and that gets you optimal lift to drag. Thus the power is lower.
We used the rotor rpm at 96% to gain performance (we were not in trouble with too little performance, we uised it to get more than we had planned.) To qualify the rotor at the lower rpm, we tested all the way to Vne that way, at 96%.
One reason why we could use Nr that way was that the vibration absorbers were effective axcross a wide rpm range (the VTA and Bifilar are great that way).