Sitting at 20 I'm all about trading back. 1st rounders work out about 50% of the time.
2nd rounders aren't actually that far behind.
The more picks you have the more chances you have of getting a player that will make your team better. SK traded back a couple of times last draft and the extra picks netted us Watford, Okafor, Juke and Swope. Well technically 2 of these player but the point is even though Swope was a miss Juke was a home run. Let's see how Watford and Okafor turn out but I would bet one of them is starting in a year or two.
Billecheat has been doing this for years. Now if Barr or Mathews or Mack drop to our 20 slot then stay put and take the gift. Point is they have to love who they take at 20 or should move back.
Now this is where Jeff Gollin (haven't seen him post in awhile so I hope he's okay) would explain about having 5 or so guys rated about the same and so you should only move back at most 4 spots.