I don't use accents in my characters though the best advice for those that do is only to use a "flavour" of what that regional accent is like - say the odd word now and again with the meaning clear from context. This is mainly to avoid speech in characters being unintelligible to those who aren't acquainted with that accent. I have come across "glossaries" at the back of certain novels and must admit I don't like these. I don't want to have to look up the glossary to understand what the characters are saying.
With regard to swearing, I will get my characters to do so if appropriate. None of them swear gratituously (though I recognise that can be a major trait in itself and reveal a great deal about the speaker). Sometimes I report swearing - for example Character A turned and faced the walll and swore for several minutes. Character B was not surprised. I do this partly due to my personal tastes and faith and I am also all for the reader being able to fill things in for themselves. I love doing so myself when reading other authors' works.