The rear end of the ball is the most important. That is why we do not load the ball with the sprue down. Marks on the front end of the ball or bullet will have little to no effect on accuracy.
That said, I use concave rod and concave starter tips automatically. These sometimes leave a ring-mark but it does not appear to hurt accuracy, indeed, if anything, it might help in card and string cut competitions.
HMann's experiments years ago determined that with properly stabilized bullets, badly marked and deformed 'bullet' tips had no effect on accuracy, but even minor nicks on the outside edge of a bullet's base, caused extreme accuracy changes.