with Strasburg and Zimmnn in the rotation we will (and should) have a lower IP/G than the top teams. I wouldn't worry about that too much.
i think it's a pretty important stat, and our low IP/GS was one of the reasons why Rizzo got Jackson & GG, both are 200 IP pitchers. A high IP/GS keeps your bullpen fresh over the course of the season. Look at recent history:
2011: PHI, AZ, LAD, SFG, STL, MIL (PHI, AZ, STL, MIL were playoff teams) (Nats were 15)
2010: PHI, SFG, AZ, STL, ATL, CIN (PHI, SFG, ATL, CIN were playoff teams) (Nats were 15)
2009: STL, ATL, SFG, AZ, CHI, COL (COL, STL were playoffs, PHI was 8th, LAD was 10th) (Nats were 13)