I am using an old Trek 520 Touring bike frame for cyclocross. When I put cyclocross tires on my 7 speed wheels, the tires brush up against the frame just behind the seatpost. As a result, I noticed that the rear wheel is not centered on the frame (relative to the seatpost - its substantially to the left, non-drive side). Initially I thought this was a dishing issue. However, I put a 9 speed wheel on the bike an it too is off to the left.

Is this normal for a frame (the drive side chainstay does bow out, so the back triangle isn't symetrical - but that would suggest the wheel would be centered to the right, not the left?), could the frame be bent (the paint is fine and there are no ripples in the steel to suggest it being bent), am I being a pinhead about something that is, in fact, correctly installed?

Thanks for any insights here.