I'm against this on principle - when I go back to edit something I've posted, I expect to see the text I entered, as I entered it. The Markdown renderer, and then the renderer in the users' browsers will transform it into something very different, but I like to know that what I typed is still there, behind the scenes, somewhere.
That said, whitespace is really small potatoes. And I don't use tabs anyway, so... y'know... It wouldn't effectaffect me, screw everyone who does.
...If there's actually some tangible benefit to it. You're talking about re-indenting someone else's code - this is actually harder when the original text is indented using spaces, since you can just tell your text editor to expand tabs to n spaces, where n is whatever your preferred indentation might be. It's... kinda the only advantage tabs have going for them.
Of course, if you really want to make your life easy, you use a pretty-print utility.