Regarding rifles fired from good machine rests.....
It shouldn't affect the accuracy on target too much as long as the hold is consistent, because from a given position and with a consistent hold the recoil behavior of the rifle should be pretty consistent too.
Right you are. In my opinion, recoil behavior will be extremely consistant and won't affect the rifle or ammo's accuracy at all.
A friend shot a dozen or so 10-shot groups testing his Win. 70 based match rifle clamped in an identical machine rest as I posted above at 600 yards. The 308 Win. ammo groups were between 3/4ths inch and 1-1/2 inch. He then shot a 40-shot group with the same ammo that went into 1-7/8ths inch. When he hand-held the rifle slung up in prone shooting the same ammo, his groups were in the 7 to 8 inch range on target at the Nationals. He won all three of the 600 yard matches that year. Not too shabby for full length sized cases either.
The best competitors shooting prone hold their aiming area inside about 3/4 MOA. Then try to break their shots inside 1/2 MOA. Such is the way things are when our pulse beat wiggles the line on a Richter scale of vibrations. Best I've observed was in 1997 watching Corky Tyson set the 600-yard prone record shooting all 20 shots inside about 4 inches using aperture front and rear sights.