I shoot split...kind of. I put very little pressure on the index finger. As Rob said, it is more to control the nock fit than to draw the arrow. Again, as Rob pointed out, the middle finger is aligned with the forearm. By putting most of the pressure on the middle finger, I get cleaner releases without the noise that seems to go with three under. It also tends to minimize plucking the string (my nemesis). Another benefit is it keeps me from wearing out my index finger. I've dislocated the first and second knuckles of my index finger, which has left it pretty crooked. If I try to put equal pressure on my index finger, I blister the side of that finger.
I can and have shot both ways, but I just feel that I have better control with the index finger above the arrow.
Shoot whichever way works the best for you. There is no wrong way, just various right ones!