If I'm posting in a thread where I'm assuming everyone who's reading the thread has already seen the thing about to be spoiled but no one else has given major spoilers yet I usually put a note at the top of the thread saying something like, "I'm assuming everyone reading this far into the thread has seen the episode, so no spoiler blocks below," just in case.
Most other times, though, I use spoiler tags or spoiler blocks. My general rule is that if it's more than a paragraph I use an sblock, but if it's only a few words or a single sentence I use the spoiler tag.
There was one time, though, when I was discussing the first X-Men movie long after it had been out in another thread about other X-Men movies (I think it was discussing a really early-release trailer for most recent Wolverine movie) and I used spoiler blocks just in case, but somebody yelled at me (okay, it wasn't yelling

) saying it wasn't necessary to sblock secrets for a movie that was over five years old. Personally I feel like it doesn't matter how old the movie is, if you're considerate and you think there's a good chance people reading the thread don't know the material you're talking about then use an sblock.
I know I really HATE if I accidentally run across a spoiler for a movie or book I haven't seen/read. I feel like assuming everybody has seen the movie or read the book just after it came out (even if it was a very popular book/movie) is not fair to people who are a little slow to pick up on trends or who don't have the money/time to go out and see every movie as soon as it is released. Some people don't realize they want to see a movie until years later, and it's nice if it hasn't been spoiled for them by that point.
(By the way, for those who just make their text black as a spoiler - don't do that! Use actual spoiler tags. Making the text black does absolutely nothing if somebody is reading the thread using the white skin.)
And speaking of spoilers... very few people I know have had the ending of Fight Club spoiled for them, even if they saw it years and years after it came out. It has been suggested to me that the movie itself has a subliminal message not to give away the ending. "The first rule of Fight Club is you do not talk about Fight Club."
