The rule is: DON'T
if a game was released too recently, or worse yet, it's unreleased, use the Spoiler tag to wrap all the offending content in the body text, and don't type any spoiler into the title. Properly indicating there are spoilers inside the post is very recommend too. Oh, and as suggested by some people, major story spoilers from old games should be avoided as well!
What is "too recently"? Basically, use your common sense for this one. If you think there is still a lot of people playing the game for the first time, then that is a good indicator that you shouldn't be saying out loud that Darth Vader kills Dumbledore and things like that.
Thanks for your understanding.
Edit: and this new rule has already been edited, thanks everyone for the suggestions.