I've never really seen a spec on this. Usually I just leave it factory, but I don't really know if even the factory is doing it consistently. Flipping with only the upper flipper while keeping the lower raised is called "staged flipping" and can be really useful during multiball. For your own games, I'd recommend making the gap a bit wider if needed so thay you can make use of it. Only make it narrower if you're purposely trying to make the game more difficult.