Like all "rules of thumb" there are exceptions. But generally QB, OT, CB, and D-linemen cost more than S, LB, RB, WR. Therefore are generally taken earlier in the draft.
Ted Thompson really is looking for best player available and the long term interests of the team.
Woodson and Harris are over 30. Woodson has some injury history and played hurt last season. With Dendy, Bush, and Blackmon in development, CB will either be addressed with a high quality pick or not at all.
So much of what Ted does depends on
a) Who is on the board when he selects and
b) are they worth it
c) if there is a lot of value on the board(meaning several options) he can decide to trade down.
He truly cannot predict what he will do as it really depends on how his board looks at the time to select. If a top CB or RB(Lynch) is not available I believe he will trade down as there are a lot of good WR's in this draft that one can be picked up later. RB's like Bush and Irons are not worth the 16th pick.
Bottom line- If 2 CB's and 2 RB's are taken he will trade down. Otherwise he will pick one of those 4.