On turbine powered helicopters the rotor RPM is automatically taken care of by the engine computer; if there was an increase in rotor drag this would result in the torque increasing at a constant collective pitch setting, rather than the rotor RPM decreasing.
From my own experience, it doesn't happen when operating below Vne, as I always do.
Retreating blade stall isn't the only issue beyond Vne. The phenomena is usually preceded by suddenly increasing vibration levels, which do the aircraft harm over a period of time. Vne is set to take this into account. A helicopter designer calculates the strength of his airframe and components to cater for vibrations caused by time spent in the hover, climb, cruise, descent, autorotation and high and low speed flight. He assumes percentages of total flight time that the average aircraft will spend at each stage of flight.
To operate outside the design envelope, or spend long periods close to the edges of it is therefore unwise in the long term as well as in the short term.