I don't think it matters really. The most important thing, is not to let any swinging occur, which will allow momentum to aid in the lift. Otherwise, just get up there.
I normally look up and arch until I can feel the back kick in. I think most of the swinging comes from dropping from the up position instead of releasing the back to move down for the next rep.
I like to look up at the ceiling with an arch, concentrate on a point in the ceiling, and focus on bringing it closer to me. I know it will never reach but helps stay steady throughout the rep. straight up and down. ceiling in and out.