Is this a sideslip to lose height quickly or a crosswind approach technique?
If it is a crosswind approach technique, then I personally don't like it as it contains the ingredients for a good prang: low, slow and out of balance. At least with the crab technique you aren't getting into a potential spin situation on final. The aeroplane doesn't know there is a crosswind, why cross it up.
I've seen cessnas placarded with sideslips not to be performed with flaps extended. I'd be most annoyed if people sideslipped my pipers with or without flaps. The only good reason I can think of to sideslip an aircraft with flaps is if you need to get into a very small paddock, and you really know what you are doing.