Tight or untight is personal preference. Personally, I hate falling from an x-height read down into the deep chasm of a space before I hit the smooth slide across an em dash just to face another deep drop before I continue.

Each font reacts differently, so resolve comes individually for me.
SDK
Susan Kirkland
SDKirkland Master Designer
Author of
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While at some level it is a matter of personal preference -- there are some common conventions that one violates at considerable risk. The British, for example, use a word-spaced en dash for the dash that signals a break in the thought within a sentence. In the States, a closed-up em dash is usually used for this purpose -- see, for example, the CHICAGO MANUAL OF STYLE.

Having said that, we typically use a dash 3/4ths of an em long centered in an em space (a dash of 750 units with left & right sidebeaings of 125 units with a 1,000 unit em) for the "em" dash. With really condensed fonts, I'll make the dash *about* the width of a capital M, and the sidebearings won't be 125 units. I've done this for over 10 years now, & no publisher has yet complained.

With en dashes -- usually used with numbers -- I'll usually make up 2 dashes, one to center up (vertically) with uppercase (lining) figures, and one to better center up with lower-case (oldstyle) numbers. Usually only one em dash, though. I suppose one could make a slightly higher dash for capitals with OpenType fonts, just as one makes up extra parens & brackets for full capitals.

Not that it's particularly relevant but, in the olden days when setting type meant staring at a stunning amber (or green) screen full of monospaced characters and trying to keep a mental picture of where you were on the page, we used to insert a thin nonbreaking space to the left, and a thin regular space to the right of an em dash. The rationale was that if the line had to break in the vicinity of the dash, it would do so after, rather than before the dash, or--if there were no spaces at all--carry the dash and the words either side of it to the next line, which sometimes made for excessive white space. Nowadays you can actually see what you're doing, so it may be a moot point, but we old-timers are inclined to spin yarns about the days of yesteryear at the least provocation.

CMS rules are rather strict: no spaces around any dash. For what, you ask? My Chicago-born copyediting girlfriend says it saves space. Very Midwesternly efficient... and stuffy if you ask me.

I've held the University of Chicago in contempt for far greater impositions, such as exporting their faculty to the Art Institute of Chicago to suck the life out of foreign cinema courses.

I'm a half-space kinda guy for emdashes in text settings. I've taken to setting my sidebearings for such a preference when desgining my typefaces. Just my way of getting back at U of C, I guess.

Raising the endash between lining numerals is a valuable lesson as well. Thanks for advocating that one, Charles. With Opentype, such automatic adjustments are easily within reach.

I find it depends on the typeface.

I really like ultra-fine emdashes that come right up to the adjacent glyphs, but if they're not fine, they need some space.

such as, please?

f***in aye... feel it out.

If you have to ask, you'll never know.

go buy some fonts.

Times: thin dashes, fat hyphen.
Minion: moderate weight dashes, same thickness hyphen.
I'm sure you could find a similar variety of "dash strategies" in your font library, Tina.

I apologize for getting saucy there. Foul language is as counterproductive as hand-holding.