It depends on the bump map, and what you are trying to achieve. I prefer to set the strength to 100% and adjust values for min and max (which are in cm - black will match min (negative) setting and white the max (positive) so you need to guess how large the relief would be on the surface. For skin, if the bump map goes all the way from white to black, you are more likely to want to narrow the range than widen it but if most of the detail is in a narow range of tones, with the high-contrast areas used for scars or pimples or other high-relief elements then you may need to increase the spread between min and max. It's also a matter of taste, and of the kind of texture you are using (painterly vs photoreal, for instance).