Cleaning will reduce voltage drops due to resistance.
Lower voltage at idle is generally not a resistance issue. Its a lack of power generation at idle rpms. Resistance to current feeding the rotor (and in the return) can contribute to this, but if its really low then its worth troubleshooting.
If its resistance, that will show as voltage drops in the circuit current is flowing in.
If its at the alternator, do a full field check to see if its the alt or the VR.
If its the alt, then check for blown diode(s) or one of the windings is open or shorted. Lose any one of the 3 stator or rotor windings, or the rectified output from same, and power production will drop accordingly.