em-dash and spaces

In a comment to my post the other day about my current prompt challenge, Steven said

When setting em dashes the typesetter should not place space on either side of the dash.

When I started editing the PCBA Ampersand and put together a style sheet, I asked several people about em-dashes and spacing, and got several different answers. I decided against spaces. And for the haiku in all my calendars, I’ve not used a space. But Steven’s comment made me notice that wordpress (which I use for this blog) has its own idea about dashes and spaces. It turns a double dash into either an en-dash or em-dash, depending on whether the dashes are at the end of the line or have spaces around them.

• Double dash — with spaces; wordpress makes an em-dash
  
• Double dash–with no space; wordpress makes an en-dash
  
• Double dash at the end of a line, with no space, but becomes an en-dash–
  
• Double dash at the end of a line, with a space, but becomes an en-dash —

One solution is to always use the HTML character for em-dash (—) instead of counting on wordpress.
A quick look at some style guides here on the web gives a variety of answers to the spacing question. Some say it is optional (Yahoo), no spaces (Chicago Manual of Style), spaces (Associated Press Style Book). And the British, in the Guardian style guide, ignore the issue by saying “Dashes should be n-dashes rather than m-dashes or hyphens.”