I've got a few kilts that have the center of the apron as being in the center of the tartan square, but most center on a prominent stripe. This would seem to be the choice of the kiltmaker and would be hard to change after that. Although if the tartan is small, a small skew to one side or the other would be possible. Then the "gig line" could be shifted.

I prefer the center to be on a stripe.