I'm not an expert on these things, but I'm not sure what it would accomplish. My understanding is that a browser loads the html, then makes requests for anything else that has to be loaded (javascript, images, etc), but makes a bunch of those requests in parallel. So even if you were to use the above method to control the order in which the ads were requested, I don't think you'd be controlling the order in which they displayed, because the one that loaded the fastest would still show up first.
Thanks for that. According to Google it will make a difference but can anyone else shed more light on the matter? Don't want to go to the trouble of trying to sort this if it's not going to be worth it?