I think that everyone's opinion on this depends a lot on context. Those living in Scotland, especially the Highlands, seem to be less inclined to wear another clan's tartan. I myself have no problem wearing my Lyndsay kilt around Edinburgh, and will likely wear it to the Royal Highland show as well. But later in the summer when I trek to the games in my ancestors' home town, I'll go in jeans. Part of the reason for that is because that trip will be a lot about discovering my Scottish identity, and perhaps developing or deepening a clan identity, so why show up dressed as something I'm not? Later in the summer when I go to attend games in California I'll wear whatever I have. Like I said, I think it's a lot about context.