Lower, and you have more control over the ball as it descends. Higher, and you bypass the first two rows of holes more easily. It's all in what you enjoy. Certain games, I prefer a faster game, especially the six cards. For a Magic Screen, I like more thinking time and control.
The manuals for the older games said to level the cabinet completely parallel to the ground, then turn up the back legs 1/4" or so. That would be very favorable to the player.