I don't know what the different rule on Grand Slam would be, but frankly, Grand Slam, IF IT'S SET UP RIGHT AND LEVELED.... (/sigh)...is such an easy game, I really don't know why you'd ever want the add a ball. On mine, I can get 30-40 runs pretty easily. I haven't waxed it yet, so there is that, but I don't see it making a significant difference. It's already clearcoated.
Though, I will say that Card Trix is way better than Spin A Card (or whatever the replay is named..). You NEED extra balls in that game to roll the machine.