At somewhere around 20-22% grades I have to stand or the front wheel starts coming of the ground. On my folding bike with small wheels and short chainstays, I reach that point on lesser slopes. Other than those steep slopes, I will stand on short steep hills to just power over them without slowing down. But on longer hills I mostly sit. On a cold ride home the other night, when my feet were pretty cold, I discovered that I get better blood flow to my calves and feet when I stand. I guess it makes sense, that having my knees and hips straightened out would make it easier for the blood to flow. I think I also use the calf muscles more when I'm standing, which would make them pump more blood. Standing also stretches out the front of my thighs, which can feel pretty good at times.