I almost always wait on QBs. The reason that people take RBs in the first two rounds is that there is a large number of tiers and great differences between production among those tiers. In today's game, there are not that many RB situations that are featured backs without question marks.
For QBs, I feel that there are fewer tiers. There are just a few QBs that will consistenly put up big numbers (Peyton, Carson, Bulger, Brees?). After those guys, the difference between the next 10 QBs in negligible.
Let's take a look at Tom Brady. His ADP is ESPN leagues is 19.4, better than Chad Johnson, MJD, Holt...and just below Travis Henry and Marvin Harrison. By taking Brady there, you're sacrificing at RB and WR. If you took Kitna at ADP 100, you could have Chad Johnson at 19. The WRs are 95 are Isaac Bruce, Vincent Jackson, Bernad Berrian, etc. Last year, I have Kitna at 13.56 PPG and Brady at 13.44 PPG. Tom Brady and Isaac Bruce or Kitna and Chad Johnson (or MJD)?
Personally, I stock up on RBs and WRs in the early rounds. When you have several solid RBs and two top WRs, an average QB will win it for you. I won a league last year that started 2 QBs drafting Culpepper and Volek as my top two (ended up starting VY, Pennington, and Losman most of the time). Top 4 picks were SJax, Holt, Roy Williams, FWP, Driver.