Uh... Bogie here... Graphic Designer and Typographer
And I learned to type on a real typewriter.
The reason to not use two spaces after a period is that it makes things go weird when you justify text - when you have it line up even with both sides of the column. the double-spaces tend to cause "rivers" of white to run through the page. You may have noticed them at one time or another when looking at stuff, or maybe not. I know I'll notice them immediately.
Type how you want. But if it gets published, someone's gonna run a set of macros on it, turn all the double-spaces to singles, all the straight quotes to curly ones, double-dashes to em or en dashes, etc., etc...