Originally Posted by pholy
Somewhere in my net reading I saw that the n-dash was used for numeric or date ranges, without spaces; the em-dash, without spaces, everywhere else.
OTOH, I can understand having more uses for the en-dash... I think you should be consistent within a work, though.
Whether one uses em-dash with no spaces or en-dash with spaces for "any break in thought" is a stylistic choice, and as with all such arbitrary choices, the 'right' choice changes with time and place.