Absolutely, if I missed a show, I will avoid any thread or site that may spoil me on it. Naturally, people are going to talk about it after it's aired or even during... that's at least half the fun!
My only exception is for movies. I won't spoil even a classic movie for anyone. I try to keep any information about a movie in a thread just for movies (that I assume people would know would contain spoilers about the movie!) Otherwise, I would never mention any major twist in a movie in any other thread. I've noticed other sites are pretty respectful regarding movies as well. They will generally let you know if a spoiler is coming. I guess the difference is people will check reviews before they watch movies, but there is no reason to check a thread on a tv show before watching it.