I generally stand; I find it a lot easier because I can shift my weight on the pedals to move forward/backward rather than having to provide all the force with my muscles alone. Also, I find it easier to trackstand no-handed while standing than sitting, for the same reason (it's pretty straightforward if you hold the saddle with the insides of your legs).

marqueemoon: I haven't actually tried seated on my road bike, but I usually find standing pretty comfortable (especially since I'm usually geared lower than my fixie when I come to a stop). On my road bike, I generally keep my legs fairly straight and modulate pedal pressure by flexing/extending my forward ankle.