When I got my CFI rating in 1981, instrument flying in helicopters was very rare. In fact, back then very few companies required it for employment as a commercial helicopter pilot. Today it is much different. An instrument rating is required for most jobs and all professional training programs include the rating. Moreover, any pilot who begins their career instructing (as most do) will need a CFII.
Thirty years ago instrument flying was airport to airport. Now with GPS, approaches can be designed inexpensively to almost anywhere, making IFR helicopter operations more reasonable. However, there are some limitations with helicopters. One of them is range. Helicopters typically carry about 3 hours of fuel. With a large weather system it can be very difficult to fly to your destination, find a qualifying alternate and maintain the required fuel reserve.
Because of the unique characteristics of helicopters, the CFRs provide some help with the range limitation. Helicopters are required to have a 30-minute fuel reserve as opposed to 45 minutes for airplanes (CFR 91.167 (3)). Regarding the weather at the destination used to determine if an alternate airport is required, CFR 91.167 (b) (2) provides lower minimums for helicopters.
(i) For aircraft other than helicopters. For at least 1 hour before and for 1 hour after the estimated time of arrival, the ceiling will be at least 2,000 feet above the airport elevation and the visibility will be at least 3 statute miles.
(ii) For helicopters. At the estimated time of arrival and for 1 hour after the estimated time of arrival, the ceiling will be at least 1,000 feet above the airport elevation, or at least 400 feet above the lowest applicable approach minima, whichever is higher, and the visibility will be at least 2 statute miles.
Also, under CFR 91.169 (c) (1) IFR alternate airport weather minimums are different as well.
(i) For aircraft other than helicopters: The alternate airport minima specified in that procedure, or if none are specified the following standard approach minima:
(A) For a precision approach procedure. Ceiling 600 feet and visibility 2 statute miles.
(B) For a nonprecision approach procedure. Ceiling 800 feet and visibility 2 statute miles.
(ii) For helicopters: Ceiling 200 feet above the minimum for the approach to be flown, and visibility at least 1 statute mile but never less than the minimum visibility for the approach to be flown.
The regulatory relief has helped helicopters use the IFR system more often.