For those looking at Tartans other than Military, or Ladies looking at "Dress" tartans, here's the rule:
It was during the Great Scottish Revival that the tartan / clan association was enforced in that only the chief of a clan could allow his clansmen to wear the clan tartan. There is no evidence to support that such a rule existed pre-1746. In our modern era one does not have to wear the tartan of their clan; the Lord Lyon of Arms, the chief Herald of the Kingdom of Scotland, has made the determination that the wearing of the tartan of a clan means that you are showing your support for that clan and nothing more. While it encouraged that one wears the tartan of their clan, one can wear any tartan they so choose, and you no longer need the permission of the clan chief to wear the tartan of that clan.