i dont remember where, but i read somewhere that i should use index finger on rings, because it creates a shorter distance to press. i'm not sure if this matters much or not in the long run..
I use middle on the rings. It would be optimal to use index on the rings, because thats the maximum width and you would have the shorter stroke; however, this isn't feasable for everyone. You have to have adequate shoulder and lat strength to use the index on rings. This is why you see a lot of people with a slightly narrower grip.