The instrument illumination rheostat serves two functions (speaking for a 124) - it directly dims the instrument illumination bulbs and 2) dims other internal illumination through a control unit.

In any case, you needn't worry about doing any damage by jumpering the rheostat. All you're doing is bypassing any extra resistance (due to oxidization, etc.) that may have built-up over time between the wiper contact and the resistive element. The end result is the same as turning the knob of a new rheostat to the full On position.