What could be discussed, in thread about "HP the movies", without spoiler, if we consider that nothing should be told about the books? It's not because the third film has been released that previous films could be discussed (after all, someone could still not have seen them, as much as he could have missed the books).
Current film: reveals "things" about the current film => spoiler tag
Past film: might spoil people who didn't see the film => spoiler tag
Past books: might spoil people who didn't read them => spoiler tag.
At some point, any HP (or anything, it's easily replaced by anything...) thread will need to include the spoiler tag. Therefore, it's better to make the presence of spoilers the default. In a thread about a HP film, the reader should assume information about HP will be discussed. Same in a thread about having a campaign set in Hogwarts.
It's not the same thing as revealing that Saving Private Ryan ends with the private being saved in a thread about AC calculations.