It helps me. My setup is a little non-standard; I'm still using cameras on tripods rather than having camera mounts attached to the base of the scanner. That came in handy recently, when I had a book that was wider than it was tall, and needed to be shot in "portrait" mode rather than in "landscape" mode. I made another grid for that.
Anyway, having the cameras on tripods means I set them up every time I'm going to shoot a batch of books. I use a tape measure to make sure they're the same distance from the platen, and CHDK enables me to make sure the zoom level is the same on both cameras. At that point, I just try to get them both positioned within the same grid lines. If the page doesn't line up with a grid line, I know something is wrong -- either the tilt of the camera is off, or the tripod is off to one side and needs to be centered.