No, nothing else is needed. You want a "dry heat" like a grill or smoker would deliver. Lots of fat/fluid is released, so they are in no danger of drying out. I set up my slow cooker as a low and slow "oven." Most crock pot rib recipes call for some liquid to be added at the beginning of cooking—a can of Coke, BBQ sauce, water, or whatever. Then there are cut-up onions or many other things. It is all just so wrong; we should not "parboil" ribs or cook ribs in liquid.