U.S. Copter GPS SIAP's have tight design constraints as they are generally to hospitals or other special use facilities in urban areas where numerous obstacles are present that impact the approach design.
These constraints include:
--the TAA "T" leg length is generally only 3 NM;
--no segment of the approach may be flown in excess of 90 KIAS;
--final and missed approach segments max airspeed is 70 KIAS.
The latter is a fairly demanding criteria since three-axis autopilot systems are generally limited to a minimum speed of operation not much below 70 KIAS.
Another factor is that these procedures are routinely performed Single Pilot.
The technique I have found works best for accurate flying of the approach procedure and smooth ride for the passengers is to select IAS and establish 70 KIAS prior to arrival at the IAF, controlling altitude with collective. Subsequent altitude changes/rates of descent, up to and including descent to the MDA, are controlled with collective while closely monitoring altitude as the primary parameter.
Smooth but positive collective reduction for descents and leading the altitudes appropriately to level off at step-down altitudes and MDA will establish the helicopter on assigned altitudes while minimizing the cyclic pitch changes necessary to maintain the selected IAS.
If a missed approach is required, simply increasing collective will establish the helicopter in the climb at the prescribed 70 KIAS for the missed approach segment.
In summary, for routine instrument approaches to airports, i.e., ILS or non-precision approaches where other traffic may be a factor and approach design criteria is more expansive, altitude preselect modes work best. However for Copter GPS-constrained approaches, IAS seems to work best in my experience.