I believe the POH for most PA28's state that you should NOT sideslip with extended flaps...[I may be getting mixed up with a 172, haven't got a POH to hand].
You should be able to get a good rate of decent in a slip in a PA28 without exceeding any design limitations. Can also be useful if you have other problems, for example wing fire [unlikely
], trying to put an engine fire out or can't see out of the window becasue crap is coming out from under the bonnet.
Some Emergency decent procedures are specific, for example the P28R emergency decent is basically cut throttle, prop forward, flaps extend, gear down, pitch down to top of white arc and decending fairly steep turn. This will get you down very quick.
Check the POH for your aircrafts emergency decent procedure...
Cheers
EA
