I'll second what Gary has said. The golden rule is - If it hurts while you are playing, you're doing it wrong. Ideally the wrists should be relaxed and elbows should be above the keybed for a smoother playing experience.
For me it's just what feels comfortable. I know that when you classical piano the teachers tell you elbows just above, almost parallel, to the keyboard but I tend to have them a bit lower sometimes on the Hammond and just a bit higher or parallel on the piano.
So I guess it is just what feels right to you. As long as you are comfortable I would say that there is no "wrong" way to be positioned while playing
Its quite a subjective issue. People have different hand sizes and finger length. If you have short fingers (like Elton John) your posture will be quite different to a player with long fingers (Horowitz/Rachmaninov)
It also can depend on the style of music played.