In the 747-400 we retract flaps to 20 on the go-around in all cases -- flaps 25 or 30 approach/landing, 3- or 4-engine approach.
I wouldn't assume the procedures used in a 747 would be applicable to a C172. Doing so could kill somebody if they blindly retracted the flaps at the wrong time and either stalled or sunk into the ground.
I would not trust an instructor who did not follow the Flight handbook / Operator's Manual for the airplane in which he was instructing.