Cloth *does* have a "good" side and a "bad" side. Sometimes when you get the cloth in from the mill there are slight pulls and knots on one side of the cloth. You want to make sure that that side goes on the inside of the kilt. The "good" side (or face) of the fabric will have the twill running from lower left to upper right, when the kilt is being worn.

But with modern fabric production techniques, I only rarely encounter visible flaws in the non-face side of the fabric. Sometimes it's so flawless that there really is no difference.

If I had a kilt made with the twill going the wrong way, so long as there were no visible flaws in the cloth, I don't think I'd give it a second thought. I'd probably be the only one to know the difference!
Matt