typesetters in the past set a bit of extra space between sentences, so the separation is clear. i support that, as i think it makes it look nicer.
now some people try to get this nicer look via the old typewriter-trick of putting 2 spaces at the end of a sentence. but that doesn't work when the text is turned to .html. and further, the extra spaces mess with our editing process, where we often search the text for 2-spaces, just to make sure we haven't inadvertently introduced extra spaces in unwanted places.
so the best tactic to use these days is to put only one space after a sentence, and hope browsers will soon support better typography.