Lets me start off by saying you would never shoot photos this underexposed or overexposed unless you did it by mistake or you really feel it looks good, after all, it is your creation.
That being said, I created this video to show you that you can get much more information if you underexpose a photo versus overexposing a photo. Once you blow out your sky you can never get it back. (watch the vid)
I always expose for my sky, this will guarantee that I never loose it. All the shadow areas can always be brought out in post.