My two pence as a Marine vet(still serving, just air force now!)...

Wear it respectfully, and if I see you in it, I will shake your hand. Dishonor the colors, and we will have a chat. While some things military should not be worn by those who have not served, i.e. a green beret for instance, the tartan is not one of them. I like the fact that non-vets want to respect the finest miliary branch in the world.

Birddog