Can only speak for the 737-NG's
These have engine driven and electrical hydraulic pumps. As soon as the engines are turning, hydraulic pressure will be available, so generator selection is irrelavant. You must fit the steering pin for a push back.
Like any system, this can be Murphied. I have read reports that ground crews have used the wrong diameter steering pin or placed it in a rigging hole instead of the lock out hole, thus failing to properly lock out the steering.
