I don’t think spoiler tags are necessary. It would be bothersome having to insert them into every single article that mentions plot or ending details. I like it the way it is now, where we can just get on with the job of compiling information without worrying about spoiling things. Most Rayman games are quite old at this stage, and their story has always been secondary to their gameplay, so spoilers aren’t really a pressing concern for them. Spoiler tags might be useful when Rayman Origins is released, but if you haven’t played Origins and you don’t want it to be spoiled for you, why would you continue reading a paragraph that begins with the words ‘In Rayman Origins...’? I think readers of the wiki need to assume a little responsibility for themselves and their own enjoyment of the games, rather than relying on us to take care of it for them. Looking up new games on a fan-wiki before you play them isn’t very clever if you want to be surprised by the plot.
Wikipedia used to have a spoilers tag, but they got rid of it. I suspect we would eventually end up doing the same thing.