When should the half bank mode of an autopilot be used?
Seems like the accepted practice is to use half rate when faster than 250 knots, in cruise, or when flying in icing conditions.

The AIM (5-10-3) also states that pilots should use half standard rate (through probably referring to hand-flying) when on final of a no-gyro approach like a PAR. 

The rest of the time, in terminal areas and when holding, standard rate is expected by ATC. When unable to maintain standard rate (icing, for example) in those cases, notifying ATC should suffice.