I will ocasionally wear a fly plaid with my black Argyll; fancy (but not black tie) afternoon weddings, or Knights of Columbus funerals (they wear tuxes, satin-lined capes, swords, and bicorne cocked hats). On the rare occasions when I do, I wear a waistcoat. I've never worn it with my tweed jacket.

I wear a fly plaid because I live in Florida. A full piper's plaid (in addition to everything else I wear) would give me heat stroke.