The appropriateness of spaces before and after a dash depends on various considerations: the typeface used, the medium (print, online), and so on.

In the MLA Handbook, we show no space before and after dashes because that’s the standard format used by publishers in the United States. A student who follows that model will be sure of producing professional-looking work, at least as far as the dashes are concerned.

There is sometimes a good argument for spaces, however. In some typefaces, the dash is so short that it does not provide adequate separation by itself. The designers of such typefaces intend users to add spaces.

Online environments like e-mail and web pages sometimes do not permit lines to break before or after dashes. In such a situation, when a long sequence like “perspective—considering” does not fit at the end of a line of text, it will be carried over to the start of the next line, leaving a hole at the end of the previous line. Typing spaces before and after dashes is a way to avoid this.

These comments refer to projects in which the decision on spacing is up to you. In a classroom, the teacher usually has the final say on the formatting of assignments.