To clear things up, it's tolerated to an extent, as some people mentionned. Swearing is often unnecessary and can be offending to some people. This being said, just avoid things that are really borderline. If it is used to insult a user, then obviously it's wrong.

Your best & safest option if ur concerned that ur sig may unintentionally violate the site/sig rules, is to PM it to a "Members ++" prior to adding it

They will review it & inform u if it ok to add or violates a rule