As I understand it Mode S is supposed to operated all the time. The only reason Mode C isn't as the discrimination of the regular Terminal Approach Radar is not good enough to stop Mode C returns garbling. Garbling results in spurious returns and the odd emergency code activation. That mouthful-of-letters machine is better than a regular TAR, so the ground returns can be displayed, and should be.
Operate the Mode S all the time. At 'normal' airports select on/C when entering the runway. When this instruction exists on/C when the motors are running.