My take on this is just slightly different from Mr Zack's. He prefers Courier (that is, monospaced) fonts in work he is considering. That's fine; after all, he writes his own guidelines. For monospaced fonts, two spaces after the period does make sense, because the monospaced period is already artificially "farther" from the sentence it ends.
Guidelines that call for (or allow) proportionally spaced fonts, though, will probably silently wish for a single space after a period. In those instances, the period, being much closer to the letter with which it is associated (and even, in some instances, kerned slightly underneath that letter, such as a capital P), just isn't "floating" like it is in a monospaced font.
Thus, the default rule I would suggest—"default" meaning that if the guidelines say something different, follow the guidelines—is:
monospaced font (e.g., Courier): two spaces after colons and terminal punctuation, one space after other nonterminal punctuation
proportionally spaced font (e.g., Century Old Style): one space after all punctuation except four-dot ellipses, which should get two spaces
There. Have I been pedantic and rule-oriented enough to convince anyone who doesn't know that I'm a lawyer?