And be ready with your answer when the MacDonalds hail you as a cousin. Just sing the praises of their tartan and you'll be fine. (Or you could fib a bit and claim a distant ggggggggrandfather and none would be the wiser.)

But to answer your question - here in the USA there are no rules, but there is etiquette. See above.

OR - simply blaspheme about the Campbells and no one will doubt you at all. :-)