I support A and C's comment. In any case, ground power should be applied as described in the flight manual - which it is for the 172. Generally the master switch is not selected "on" until ground power is removed, but it is very aircraft type specific, so read the flight manual. If you're needing ground power, you're past the point where you are willing to accept yet another problem because something is not done right.
The proposal to use ground power on a 172 "pops up a few little red flags" for me...
Is the ground power necessary because the battery was found to be discharged? Cause found and corrected before flying attempted again? (usually master was left on - but be sure it's not another failure). Could the battery have frozen while discharged? Frozen battery = buy a new battery before flying.
Is the ground power necessary because it was run down trying to start? It should have started, was the reason for not starting found and corrected? If so, has a suitable period of time elapsed (30 minutes) to allow the starter motor to cool?
Is the ground power being used because it is very cold out? A proper engine preheat is a good idea in the case too then.
If you are the pilot starting on ground power at night, have a flashlight close at hand. When you signal the removal of the ground power plug, the electrical system will be de-energized, and all those nice cockpit lights will go out suddenly. You'll either be feeling for, or using your flashlight, to find the master switch...