Frankly, I do not believe a palm bruising start contributes to fine accuracy. Too much chance to deform the soft lead ball. On the other side of things, all that is necessary is to seal the bore. When ignition occurs, isn't that soft lead ball whacked from behind by the sudden acceleration and swaged wider into the bore anyway? IIRC the process is called obturation. If that does occur, particularly with the magnum load crowd, why is the mallet required to start the ball?
I have seen with my own experience that some rifling is just too shallow to properly grip any PRB combination, but a tighter one probably better because the ball is swaged at the muzzle into what little rifling there is. But even my Zoli Zouave shot great with a patched round ball that could be started with firm thumb pressure.
Then there are the guys who feel the need to bounce the ram rod off the seated ball with such force that a steel ram road bounces almost out of the barrel. That ball is certainly hammered into a good fit with the bore.