Mani,
Here is a way to think about this, to satisfy yourself that it is safe to NOT ground a switch that is controlling 12 volt DC power ... NEC requires metal switch boxes, and metal switch frames, that are switching 120 volt AC power because if a “hot” wire comes loose and touches the metal box, or the switch frame, and then if you touch the box, or a mounting screw, then current pushed by 120 VAC potential can flow through your body to any ground surface you are touching. While that probably wouldn’t be fatal, it could be, and it might welll be quite unpleasant.
If the same thing were to happen with respect to that switch controlling 12 volts, it is
unlikely that you could even feel it, much less be harmed. It would no more harmful than grabbing hold of the positive terminal of you car battery while standing in a salt-water filled wading pool, and simultaneously leaning against the chrome bumper ... which is to say, not harmful. ( OK, well, maybe raw flesh, but not intact skin.)
All that said, there IS risk if the switch is not properly fused, while connected to a high current source. But those risks are arcing, metal spatter, and heat. Avoid that with proper fusing, and not by grounding.
As HRKDT and newbostoncons correctly say, such grounding isn’t normally done, and isn’t required.
