OK, we've got anecdotal input that says good tartan, unlike some other woolen fabrics, doesn't need to be preshrunk. I've dug a bit deeper to figure out the WHY, and found some interesting (to me) stuff.

Here's one explanation of how manufacturers can preshrink their fabric: Link

But we all know that tartan is woven from worsted yarns, and that seems to be the clue. This article includes the statement, "Worsteds go through a process called crabbing in which the fabric passes through boiling water and then cold water. This procedure strengthens the fabric." I don't know about strengthening, but it certainly sounds as if that would also pre-shrink the yardage!

So it seems that worsted = preshrunk by definition. Considering this forum has hosted exhaustive discussions of twill direction, selvedge construction, color variations, and other characteristics of tartan fabric, I think this tidbit of knowledge is worthwhile.

I'd still be interested in confirmation or clarification from the weavers, but that would just be the cherry on top STS.