I've owned one pistol with a distinctly flat-topped slide, which was in just about all other ways similar to other round-topped guns, and while shooting I was never aware of the rib on the one gun that had it.
My awareness is of the front sight and what I can see around it in the notch, and if the target is big/close enough that I don't have to use a hard focus on the sight, my focus is on the target.
That said, I know people who are focused on a point half-way between the gun and target, focused entirely on the target, etc., so there are lots of ways to skin that cat.
For a gun that had no sights, I think a rib would be an aid to holding the gun level on both axes.