I'm going to be using a prefit with a nut because I don't feel like sending my action out and it also allows me to fine tune head space. Plus no one offers a shoulder prefit at the moment. As far as the curve of the bore goes it will influence the point of impact more from heat then the harmonics of the barrel. Harmonics of the barrel affects the point in space at which the bullet exists the barrel so if you control the time at which the bullet exists the barrel during these sine waves you end up controlling your group size. But as the barrel heats up the direction of the bore curve will effect the POI shift. I think the reason a gunsmith would try to align the bore curve vertically would be to keep the poi shift vertical as the barrel heats up plus it looks better in the stock. Just my two cents.