Not to dissuade you from wearing whatever you want, but....

The lists of septs, or surnames associated with a clan, are notoriously inaccurate and full of fantasy.

When we do genealogical research, we ALWAYS start with the most recent person, oneself and one's parents, then work our way backward, rather than picking some one in the distant path, then deciding to be descended from them, and trying to work our way forward and claim som relatinship. Likely one isn't related.

There are only two ways to discover ancestry. One is tracing the paper trail backward in time, documenting one generation before moving on the the previous one. The other, which is new and sometimes but not always helpful, is DNA testing. You might find the FAQ at www.familytreedna.com informative.

The problem with just picking a clan from a list and deciding to be descended from them is not so much that one is usually mistaken, but that it can lead others to repeat the same mistake, and that it gets passed on and on. A relative of mine did just that, and now one misinformed nephew has an expensive kilt in a clan that we have no association with, while another had the clan crest tattooed across his back. And neither are all that interested in correcting the misinformation, since it has gotten under their skin, so to speak.

Since you ask, I would say the proper form when asked one's lineage is to simply speak the unadorned truth: "My surname is Stiverson, and I don't know what my lineage is." There is nothing at all wrong with not knowing.