We did this test with petri dishes in my high school biology course where we had students wash their hands, air dry them or use paper towel dispensers or air-blower dryers, and compared all the results. Oddly enough the paper towel and air drying alone both resulted in lots of bacteria growing in the dishes. I'm not sure how effective those paper towels are - or if it's just that kids don't know how to wipe their hands!