I always wear, when appropriate. It weirds me out to think my avi is walking around without undies, unless that's something I'm specifically aiming for to do. I even hunt out low-rise undies for wearing under low-rise jeans, and I would kill to find a strapless bra with a very low back for a couple of dresses I have. I'll even flop (pardon the pun) between two different shapes with different breast settings for wearing with and without bras...