I agree with the other punctuation, but single space after a period doesn't make a writer look one bit more professional. Less so, to many of us.
Less so? That's a little harsh. A lot of writers use single space and that does not in any way make them less professional. Different from how people were taught in the typewriter days, sure, but less professional, especially since a lot of house styles call for single space?
(BTW, most online publishing uses single space because that's actually what HTML does. To insert double space you actually have to insert two &bsnp's into the document. By default, all white spaces turn into single space after being rendered by the browser. Read every single post on AW and you will find only single space after period, no matter what you do).