I've never had heartburn over someone wearing MacMillan tartan; I'm (obviously) proud of it, and really chuffed if someone else likes it as well. Several years ago I a met a gentleman wearing a MacMillan tartan tie while waiting for the for the ferry from Cairn Ryan to Larne. The gentleman wasn't a MacMillan, but said he wore the tie because the MacMillan Nurses had been so kind to his wife while she was slowly dying of cancer. It was his way of saying "thank you".

My point is, by all means wear whatever tartan you like, but you might want to read up a bit on the Clan Mac?? so if someone asks you the "why" question you can share a little nugget of knowledge.