I've been mostly lurking around here and reading the many variations of this discussion for a while now, and it's fascinating, and I don't think that there will ever be a succinct answer. You'll get some people telling you that only native-born Scots with a Highland clan surname are allowed to wear highland dress in their family tartan, and you'll get people who wouldn't think twice about wearing a tartan just because it's pretty. Mostly, though, I think you'll get people telling you to do what you're comfortable with.

Whatever you decide, I think that the most important thing is to really own it. I haven't acquired a kilt yet, but as a bow tie wearer and person who often finds himself over-dressed for many occasions, my experience is that being able to pull off any garment or outfit is about 10% fashion sense and about 90% self-confidence. If you decide to wear a family tartan without a connection to that family, be prepared to shrug off the tut-tuts you're bound to get if you get into a conversation about it with the kilt police. Joe Gondek's line about the 18th century is a good one! Or if your conscience allows it, make up a distant ancestor to satisfy nosy, rude strangers and sidestep the issue altogether.

My personal inclination is to stick to family connections, but I'm a bit of a romantic and fascinated by history and genealogy. In my case my clan affiliations are pretty far back in my tree; my heritage is a lot more English and Irish than Scottish, so I'm certainly not going to point fingers at anyone about "entitlement."