The Cessna 172R has a placard, "Avoid Slips with Flaps Extended". Now
it says, "avoid". It does not say that you can't do it. There was a
discussion in Flight Training Mag about 6 months ago that talked about
that same thing. In the Cessna 152 I always have full flaps in a slip.
I don;t truely understand why having flaps is a problem in the 172R.