>>7216629Their reasoning seems to be that your body will follow where you're looking. So looking down, especially with weights as heavy as they use will be more likely to have you lean forward and possibly fail the lift. By looking up slightly, you're going to be able to follow where you gaze is set.
Rip says that looking down slightly will give you better hip drive out of the bottom. He does over-exaggerate in the videos and the book by showing examples of people staring sky high. His point is pretty well made.
Try both and see which you like, that will be the ultimate test. For me, I started gazing up more simply because when looking down at heavier weights I started to get way more of a good morning effect going and it wasn't working out as well. Obviously, this meant I had some weaknesses in other areas which have been brought up but it really comes down to personal preference and how you feel each variation effects your lift.