The advice "You can't slip with flaps in the C 172" is another flight school urban legend that should be stomped out. There are no restrictions on slipping in the "limitations" section in any of the POH's I have seen (C 172 L,M,N,or P). The note to avoid slips is contained in the "Normal Operations" section.
Frankly the C 172 has plenty of flaps. If full flap won't get you on the desired final profile then I would suggest your approach is so bent out of shape, you should go around.
But if you are high on a forced approach you want to use everything at your disposal including slips. I am dismayed at the number of students who were told they could never slip the aircraft with the flaps deployed even in the forced approach scenario.