I reccomend against it. I got the first version of 8.1 back in the summer, and its labelled as a beta release, which I didnt know at the time, but it expires after 3 months or so. It expired at halloween a couple weeks ago, and I had to download a newer version, or live with a "download newer version" popup every time my computer accessed DirectX (its a lot more than you think). So I downloaded the newest SDK thing, and now it seems I'll have until February before this one expires. Hopefully by then they'll be close to a 9.0 release.
Its upto you if you want the absolute newest version, If I had a choice, I'd go back and keep the 8.0.