I do it quite often, to stretch my back, warm a cold hand, open my lungs up a bit and just for fun. I haven't always been able to, but I taught myself about ten years ago, and was surprised at how easy it was. I had to tuck my tailbone well in and sit really upright, to "get" it. Now I enjoy feeling how I can move around, look over my shoulder, remove my pack and jacket and put my pack back on (on a good day), and in general steer my bike well no hands. I will usually put a hand on the bars if I see someone approaching, mostly to be polite so that they won't worry that I'm about to ride into them.