I was recently pointed to an article about the history of the typographic rules for spacing after sentence-ending periods, Why two spaces after a period isn’t wrong (or, the lies typographers tell about history) – Heraclitean River:
The topic of spacing after a period (or “full stop” in some parts of the world) has received a lot of attention in recent years. The vitriol that the single-space camp has toward the double-spacers these days is quite amazing, and typographers have made up an entire fake history to justify their position.
The author has a detailed historical analysis both of practice and of typographer’s manuals, going back to about 1771, showing that the historical practice was to use an em-space at the end of sentences, and that the modern practice of using just the normal word space is of quite recent origin.
The author’s own preferences are clearly stated:
For the record, before we go further, my preference is not for double-spacing, but for a slightly larger sentence space, about 1.5 times an interword space for most typefaces. But unlike the modern single-space fanatics, I don’t judge anyone’s aesthetic preferences, nor will I try to make up fairy tales using fabricated history to convince you.
I am in aesthetic agreement with the author—I’ve always liked sentence-ending spaces to be a bit bigger than word spaces, but not a full em wide. I find that TeX’s handling of spaces (and, hence, LaTeX’s) to be quite good, in that sentence-ending spaces are a bit larger than word spaces, but are also stretchier and squishier, so that justification affects them more than the word spaces. When combined with TeX’s hyphenation, line-breaking, and justification algorithms, one gets well-set text blocks with only very rare human intervention (mainly when hyphenation has to be forced into a word adjacent to an em-dash, or when there are large unbreakable objects like math formulas).
The Why two spaces after a period isn’t wrong article is worth reading for the historical content, but the bottom line is clear—you can use either one or two spaces after periods, if you are using crude tools (like Microsoft Word, WordPress, or Google docs) that don’t have good handling of sentence-ending spacing built-in.