In my enthusiastic passenger experience, I took off from O'Hare 31R enroute San Francisco, and we applied full throttle before rolling (UA 747-200). Coming back, the pilot flying a UA
747-400 decided to take off to the north, whatever runway that is, though I know it's comparitavely short, and we pretty much made a casual takeoff without any rnwy run-up. In conclusion, I don't know why they do or don't do it.
I don't know, so this is my signature.