Recently, I made a couple of 2.5 gallon test batches (same recipes except different hops). I steeped 0.125 lbs of Breiss dextrin malt in each. That would have been 0.25 lbs for a 5 gallon batch. I could really see a difference between these batches and previous ones where I did not use dextrin malt - much better head, retention and lacing. However, it was a really simple recipe used to compare some hops. The only specialty grains I used was the dextrin malt.
But like 944play said, some extracts, Briess pale LME for example, already have cara-pils in them. The extract I was using, United Canadian Pale LME, does not. Check the extract manufacturer's web page and look up what is used in the extract before you decide.