Always Move In
Ideally you always want to be moving into the ball when you hit it. Even if you are being pushed back by a high one, you want to move back, then on your last step (or two) move into the ball. This gets your body weight behind your shot, and uses your momentum for extra power and acceleration during your swing.
Additionally, moving in towards the ball as it approaches you means you can hit the ball at a higher point, allowing you to hit a more aggressive shot.
It also robs your opponent of recovery time.
Finally, moving in towards the ball cuts off the angle, and reduces the distance you have to run to get into position. This becomes very important on the return of serve.
Killer Tennis 