I'm against having it apply to anonymous users, which consequently makes me against having it for newly created users.
Anonymous users wouldn't have an ability to disable the ignored tag like a cookie user or a registered user could (how would we track it?). Anonymous users who do not want to see spoilers, in the current system, can use a -[spoiler] tag search combined with some other tag term to get by. Not superb for plain browsing, but at least they have an option. Whereas, if we implement this, then anonymous users who do not mind spoilers will not have any options to decrease their discomfort. The normal system lets us embrace both kinds of users rather than push either away, so I don't agree with forcing it on anonymous users.
If we don't enable it on anonymous users, though, then it becomes very awkward for newly created users. After all, the point of account creation (and subsequently registration) is an enabling act - you gain new functionality and opportunities. But if the default is ignored, suddenly people go from being able to see spoilers clearly to having to sit through their dimmed status. Yes, they can remove the ignored tag if they wish, but it is somewhat disorienting. As well, it would be saying for the person "You don't like spoilers", since it's a user preference that is grouped with any other interesting/ignored tags they later add. Under the current system, we don't make any assumptions for the user. When someone creates an account, we should enable them to apply what filters they want. We shouldn't force them to have to disable filters that we guess they probably want, especially for filters which didn't affect them prior to account creation.