Wow a 6-year thread. That is incredible. I enjoyed all the various comments. What was of particular interest is that some of those that posted really seemed to care one way or another. Arguing with a publisher over an n-dash or m-dash. Not worth it in my opinion. Since it is obvious from the various posts that just about anything is valid, it really does not matter. The publishers "Bible" and the Chicago Manual of Style conlict.
Basically it all comes down to personal preference.
By the way, I use them all. Just depends on what, in MY opinion, looks good. And honestly, If someone does it differently thats fine with me.