My personal rules, for what it's worth, are very simple- I don't wear tartans with "royal" in the name. I don't wear tartans that I don't have some sort of family association with. I would wear Cameron since I had a great great grandfather who was in the New York 79th in the Civil War. They were Cameron Highlanders as I recall. I wear the Black Stewart and the Weathered MacKenzie because I had ancestors who fought in the 1745 uprising, and these were as far as I know military tartans developed for the regiments. I am trying to save up the funds to get my own family tartan and a Colorado state tartan kilt. I have no problem at all with universal tartans. For me, it's a matter of respect to not wear other clan tartans or ones with which I have no historical ties. On the other hand, the truth of the matter is that the clan tartan concept is a reletively modern one and very, very few have a pedigree older than the 1700's. No native born Scot that I have ever talked to has had an issue with non clan member wearing their tartans. In fact, I have read posts with titles like "Knock off this rediculous clan allegiance nonsense." So there you have it- rules that seem to me to be simple and sensible. However, those are my own personal rules and I see no reason to force them on anyone. If you're a Mertz and want to wear my family tartan, knock yourself out.