Spot on JohnV, I think its more on the Cessna models with the full 40 flaps as opposed to just the 30 flap versions.
In the manual for my C172N it states in big letters DO NOT SIDESLIP WITH 40 FLAP and I flew a 150 Aerobat once with 40 flap which had a placard saying the same.
I also looked this up on the net once (out of curiosity) and read a post by a bloke who tried it from a high altitude so that he had time to recover. He basically said that once it goes into the sideslip it wont come out again as the rudder effectiveness is lost. The only cure is to raise the flaps to re-establish the flow correctly around the rudder.
Not what you need at low level when trying to get into a tight field. Then again with 40 flap a 172 drops like a brick so if you need to sideslip aswell there's something badly wrong with your approach !!
If I'm trying to lose height quickly I tend to drop the flaps full and do tight weaving S manoeuvres instead whilst carefully watching the speed of course!
Hope that helps,
Regards
UA