This is the case with most positions at 29 though, right? You can maybe get the best interior lineman in the draft, but you're not going to get a pass-rusher, a D-lineman, a coverage player, or wideout that doesn't have some question: limited physical tools that suggest low upside, a lack of experience or production, or injury or off-the-field concerns. I think we're all pretty happy with Chandler Jones at 21, but he was a dude with great physical tools who'd never even had 5 sacks in a season in college.
My sense is similar to what Zmart100's charts show--that there is a very steep dropoff in talent at a few positions if you don't use a top ten, namely WR, QB and DE. (I'm a bit surprised by the steep drop-off at DT). Not surprisingly these positions are also incredibly valuable in every scheme so what you see is something like an efficient market place where the added value of a great WR/DE/QB drives up the value of players, people take chances on those positions relatively early in the draft and sometimes they work great (Chandler Jones) and sometimes they're Chad Jackson.
So the question for me when you're using picks between 20 and 60 or so is do you take chances on the high risk players (draft receivers in round two and three not knowing whether they'll be Torrey Smith or Chad Jackson or Brandon Tate or Mike Wallace) or do you make sure you hit singles and doubles with Logan Mankins and Hightowers and Heath Millers and try to fill the WR/DE/QB/DT slots with volume, scheme and (possibly) expensive free agents?
Obviously you can use a combination of the two approaches but I personally would lean heavily towards filling the easy to draft positions, trying to never stike out in that range. Which, incidentally, is what Pittsburgh has typically done.
All of this has to be contingent on the individual talent pool--I'm sure there is variation from year to year on how many good lookings DTs are available or what not.