I think they can make a slight difference, but not night and day. Torque wrench is highly recommended. Park makes a couple nice ones, also check Sears Craftsman.

Watch out for cheaper post, as they area carbon wrapped aluminum - so no benefit. The Bontrager (Trek) post are very nice, I run a Race XXX Lite on my Madone, and I've been very pleased. I also use a Campy Chorus on my Bianchi and it's an excellent post as well. I'm pushing 200 lbs, but don't have any problems.