This is true of many of the tartans I've worked with as well. For the most part, it's only really noticeable to the kiltmaker. I stand back from it and look at the overall effect. If it's minor, i just leave it. If it is more than a little skewed, however, skewing and steaming is the solution.

The process works best as a two person job, one to hold the skew and one to steam. I lay a big thick wool blanket out on a large table to start with. This helps keep the tartan from silding around while you're skewing and pressing. Have one person hold the tartan skewed, and the other one can press. I typically "over-skew" and let it spring back a little after the press.

This is also the solution for the horribly skewed tartan (more than just the selvedge) that I've gotten on occasion from Fraser and Kirkbright.