I would say the racket face is square to the back of the ball at contact for any kind of serve. The racket face may be rotated sideways depending on the direction of the serve, but it would be pretty much virtical at contact. A flat serve drives through the back of the ball, but does not hit the top and push down. Keep in mind that even flat serves have some amount of spin, usually topspin. If you were to hit the top of the ball and impart topspin, the ball would have no chance of going over the net. For kick serves, the racket face is again virtical, not open. When I think about hitting my kick serves, I actually imagine my racket face closed at contact. This isn't the case, but it's just what I picture to help make sure I get proper contact. You don't need an open face because the speed and direction of the racket head will drag the ball up to give it the lift and spin it needs to get over the net and into the box. Does this clear it up for you?