The rotor tips are the last things to ice up because of kinetic heating (IIRC it is the square root of the airspeed that gives the number of degrees C increase) but the root and inboard section can ice up very quickly. It is the assymetric shedding of rotor ice that causes most problems due to the high levels of vibration this can cause.
Another thing is, that the inner area of the rotorblades, where the blades first building up ice, is essential in the case of autorotation. Especially for S/E H/C, after an Enginefailure, you may be unable to build up enough RRPM for a sucsessful landing.