I have found the opposite. I know all about sloppy technique from personal experience. That's the stuff that saps your power and makes your knees hurt. A fixed gear really helps you understand it because there's no dead part of the stroke at the pedal end of things--if you try to go slack the cranks will push back. If you want to be able to ride without knee pain, you develop a good spin. I definitely sense this when I'm riding my roadie.
I finally got a proper ride in today on the geared bike, 2.5 - 3 hours in the saddle and who knows how many miles. I was definitely much more conscious of "spinning circles" than I was the last time I rode a bike with gears. Another data point: my straps on my roadie don't cinch or hold as tight (I figure since I can't trackstand it for crap, this is probably ok) and I really notice because I can't pull as effectively on the upstroke. The fact that this is a problem tells me that my pedaling technique is good.