Tech,
The problem with using the wipers as ice indicators (I especially like the small hose that provides the washer fluid) is that it may not reliably tell when the rotor is experiencing ice. The issue is that the rotor and the airframe are in two different environments aerodynamically, so the rotor might not be experiencing ice when the wipers show some, and vice versa. It has to do with the fact that the blades are doing near Mach 1 while the wiper is poking along at Mach .2.
I have had ice so heavy that I was descending at full power, with no ice on any part of the airframe that I could see, and I have had 1/4 inch of clear ice on the entire nose with absolutely no ice on the rotor.
The only way to tell if rotor ice is around is to watch for a torque rise with fixed collective and fixed nose attitude. Of course, if ice forms on the wipers, that is a good sign that it is time to be somewhere else.
Of small comfort, but true nonetheless, the engines will behave wonderfully in ice that will otherwise reduce performance to nil. The inlets on the various S-76 models are well suited to ice conditions by design and test.