Do whatever it says in the POH.
I don't have a C172 POH to hand, and even if I did I don't know if the instructions vary from one variant to the next. But, off the top of my head, I'd have thought that the manual would suggest a positive rotation. I think the POH for the Millenium Edition suggests 55kt for the rotation speed. (Do you really mean mph? I thought all C172s measure airspeed in kt, but I'm quite prepared to be corrected if I'm wrong.)
In my limited experience, I'd say the practice of letting the aircraft fly itself off the runway tends to apply to tail-draggers. In a tricycle you would normally rotate positively. But that's only from a handful of types, and there are always exceptions to every rule which is why I say the POH is the only place for a definitive answer.
FFF
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