I bare shaft tune, and find that I need to set the nock point 1/8 - 1/4" higher for 3 under than for split to get the same bare shaft flight.
That said, string walkers will hold the string anywhere from at the nock to several inches below the nock, and they certainly don't change their nock point when they change their string grip. And, some of them can be so accurate that string walking is banned at most trad tournaments.
That's about all I know about string walking. I guess they just don't worry about the effect their string grip has on their arrow tune, since the net result is that they shoot more accurately doing it.