The POH for some Cessnas - I believe it's only the later C172s, but don't quote me on that - advise against slipping with full flap.
I fly a 1959 C172, and the POH does contain a prohibition on slipping with FULL flap (which is 40 degrees in this aircraft). I'm therefore perfectly happy to sideslip with up to 30 degrees of flap, and some people would say that in practice it's fine to slip with 40 degrees as well. You can bet that the prohibition on full-flap slipping already has a healthy degree of conservatism built into it, so there's no need for the oft-quoted nonsense about not slipping with ANY degree of flap.
I agree that there's no substitute for reading the POH of your particular aircraft, and heeding its advice rather than listening to rumours and old wives' tales.