I don't really have any true no nos. Sure, I won't use matchlight most of the time, but I have a smokey joe at my store that I cook lunch on every once in a while, and, well, I'm in a shirt and tie. I use matchlight. You won't catch it at my house, though.
I have lots of these "half rules" for things I'll only do in a certain situation. However, if I had to have one no no, it's never say, "You can't grill/smoke that!" because you can, and it will probably be awesome!
97 18.5" Kettle (restore in progress), 08 18.5 WSM, 13 22.5 WSM, 05 22.5 Performer, 13 SJ