The racket face is slightly closed on 'normal' rally shots to compensate for the spin of the incoming ball. To hit the ball back, with spin, the spin of the incoming ball must be reversed.
in a normal tennis shot, even if the ball travels with no spin, the ball will have spin once it bounces off the court. So in a vast majority of shots you always deal with having to reverse the spin.
it has been noted that if you were to hit a dropped ball, which just bounces up with no spin, you would want to hit it with racket that is not tilted.
This is all nicely discussed in this book http://www.amazon.com/Physics-Techno.../dp/0972275908
obviously players may not be aware of any of the physics. they just know it through experience acquired over thousands of hours spent hitting the ball.