Seriously though, it depends on the situation, of which there are usually two. There are either curves, turns, and vehicles which will potentially pull out in front of me, or there are not. If these things exist, I stay in the drops so I can get to the brakes and/or make a quick turn to avoid something. If it's a long, straight descent, I put my hands on the flats slightly closer together than shoulder width, sit off the front of the saddle, put my chin on or just above the bars, and get my butt in the air.

Here's Cancellara (also) doing it, probably much better than me:


I haven't tried this position while sitting on the top tube, but I might try it out next time I have a tailwind DIRECTLY behind me coming down the mountain. Crosswinds are sketchy with your hands so close to the stem.