The Cessna and Pipers with a pre-engaged starter, I will not touch, so that rules out the C172, for me. I also think it is nigh on impossible, safely.
Why?
The starter will only 'pre-engage' with the starter ring if someone operates the starter motor (on a 172, turns the key beyond the 'both' position). Just the same as with a non-pre-engaged starter motor, in fact.
Propping from behind is actually safer, since if the aeroplane moves, it moves away from you. That's the method I use on spam cans and Cubs. On the Chippy, a firm push on the wing's leading edge will confirm the brakes are properly 'on' and the aeroplane will not move.
Of course if you don't have faith in the person in the cockpit when swinging, walk away and leave it.