Actually, youre eyes are almost never exactly level, it's your brain that creates, in conjunction with the equilibrium organ, a level picture in your head.
Just try it, tilt your head, and you'll see that you still percept your surroundings as level and not as tilted.
so +1 to this idea. There should be head movement too though, as a üerfectly steady view with just the car moving isn't too realistic either.