On all of your strokes, whether it is the forehand, backhand, volley, smash, or serve. You should have your head and eyes fixed on the contact point (where you strike the ball) until the ball leaves your racket.
Note that it is impossible to keep your eyes on the ball at impact because it's happening too fast. The goal here is to just keep your head and eyes fixed on the contact point. When you do this, you will strike the ball cleaner and also decrease the chance of mishitting.If you do not keep your eyes and head fixed on the contact zone, you will disrupt the swing path because you end up jerking your head up too soon to see where the ball is going.
As you can see Federer below, he is not watching the ball, but he is keeping his head and eyes fixed at the contact zone to prevent the disruption of the swing path to the ball.