Is it important to keep the long axis of racket horizontal at impact?

[New Daddy]

Is it important to keep the long axis of racket horizontal at impact?
(for example, this instructional website emphasizes its importance. http://tennis.about.com/od/forehandbackhand/ss/federerforehand_6.htm)

Wouldn't you be able to generate more topspin if the racket head is lagging a little at impact (i.e., the long axis of the racket is right before becoming horizontal) so that the long axis (rather than the short axis) of the racket head imparts friction?

 

[Frank Silbermann]

Look at the extremes. If your racket were perfectly vertical, the speed of your spin would be no greater than the speed with which you could lift your hand. With a horizontal racket, the racket doubles the length of your arm (not exactly, but pretend), so that whatever speed your hand rose, your racket head would rise at double the velocity. Now, add to that if the racket rotates about the wrist or the elbow (windshield wipering) -- which would provide no topspin at all (only sidespin) if your racket were vertical.

If you contact the ball with the racket between horizontal and vertical, the difference would be less, but it would still be there. It would be like a compromise between the two extremes.