Been flying 172's now for a while now and most recently the P model.
If you are doing a "normal" approach start on final at about 65kts and bring it back to 60kts over the fence.
For a "short field" approach you can be at 60kts on final bringing it back to 55kts over the fence.
If there are gusty conditions or strong winds add a few kts to the above but if it is calm or <5 or 10kts then that should do the trick. Even at 55kts for a short field approach you are still about 10 or so kts above the stall. Remember though that they do not have overly powerful engines so if you are heavy be ready with the power if needed.
EDITED TO ADD THIS BIT...
I forgot to add that if you keep the oil filled up to whatever figure is stipulated on the end of the dipstick then all you will ever get is the extra going straight out the breather. Next time you go to hop in one have a look under the cowl, near the fire wall on the pilots side and you'll see a bit of hose just sticking out of the fusulage. Thats where the extra will go. For example, if the stick say's 7Quarts, then if you have 6, or top it up to 6, you wont have a problem and your oil temps and pressures WILL stay in the green. Before I get nasty replies for this one - the manual does state that the aircraft is not to be operated with less than 5 Quarts.
This is straight out of a Cessna 172P manual
White Arc - 33-85 KIAS Full flap operating range. Lower limit is maximum weight Vso in landing configuration.
So that figure is most certainly correct. Like I said, 55kts still gives you a nice buffer over the stall.
We plan on a block fuel of 33ltrs/hr and thats normally running at about 2400rpm with more often that not full loads.
Even thought the manual may not state it I've had mag fouling before because of aircraft has not being leaned under 3000' during previous flights (Obviously should really only apply on navs or longer flights - dont scare the sh*t out of your instructor by grabbing the mixture lever on downwind
Hope some of this helps you,