At this early date i would not have a problem moving to 18 or 20 and getting an extra 3rd or 4th and 5th. I believe the talent is close to even between 15-20. Combine coming up so that thinking may change. Also the X factor is say a top talent falls do to whatever sports hernia for example. That can always change your thinking.
If only one QB is taken in picks 1-14, then I would entertain a trade down as the players I want would more than likely be gone. The extra picks could potentially become a nice start to rebuilding this defense. If we got a top rated CB or S at the end of round one and Jordan Matthews in the second, that would be a nice coup. Then go front seven in round three.
I believe last trade down was for Hampton, from 16 to 19
The Steelers traded up to draft Holmes. Interesting bit of trivia - one of the players selected by the Giants with the picks they got for Holmes was Guy Whimper.
I found a history of the Steelers trades on the site that shall not be named. When they trade down, it's often in the second round to acquire more third round picks. The only team I'm aware of that has multiple third rounders this year is the Browns. If the Steelers gave up 45 to the Browns, it would return 71 and 83 (with maybe additional picks). The multiple third rounds has yielded talent some years - Wallace, Keenan Lewis and Urbik, and not so much in others (Willie Reid/Anthony Smith).