Fairly easy for Plats, depending on when you're purchasing and how Plat-Heavy a route is. Plats get first crack at the Exit rows when the website is working properly. If you book late and there are a ton of plats on that route normally, not only will you not get an upgrade, you won't get the exit either.

That brings up the question of what routes you expect to fly most, and what your booking paterns look like. For the way I fly, Plat would be great for getting exit seats...