Simply put, the "two space" rule is a typesetter's convention. It was developed for use with monospace fonts, so that your eye could easily pick up the start of a new sentence. With modern proportional fonts the double space is largely irrelevant. Further, modern typesetting software uses interword spacing to make the display of text more pleasing. Thus double spacing can make the layout less pleasing.
So my suggestion is: use two spaces if you're banging away on your Mom's old Underwood, but use a single space in any electronic format, unless you're deliberately using Courier for some ungodly reason.