I'll admit that I'm a bit obsessive when it comes to mechanics. Bicycles included.
This is about my 2010 Schwinn Madison. I notice that when I slide my rear wheel into the dropouts (or track end) it doesn't sit perfectly centered between the stays (both chain and seat). I can get it centered if I move it about a bit and hold it as I carefully tighten the axle nuts. I measured the frame alignment as per Sheldon's string method, and I get 31mm on the button from the seat tube on each side.
The bike rides just fine. I can ride straight with no hands, and it doesn't show any other symptoms of misalignment other than the wheel not centering in the dropouts without a little tweaking.
Anyone else have this issue? It bugs me a bit, but that's because I'm obsessive and I really appreciate accurate manufacturing.