Ribs are not parallel to the bore, just as rifle sights are not parallel to a rifle bore. All projectiles fall with gravity. There is 'rise' built in to even a field shotgun. Trap guns have much rise, since they are designed to shoot a rising target.

The advantage of a rib is primarily in repeatable gun mount. How much rib you see depends on how the gun is stocked, and to a certain extent how you mount it because we have the ability to adjust ourselves to the gun (some more than others).

Your shooting style will determine how much rib you need to see. Personally, I like a high shooting gun, so I never have to 'cover' the bird except on incomers. Not important how much rib you see, but that it's consistant shot to shot.

"The price of good shotgunnery is constant practice" - Fred Kimble