I hear this fairly frequently. My experience is that this entire idea is founded upon a potential minuscule difference, and relates far more to brass consistency than any other factor. What does this mean? It means that quality brass, in a carefully cut chamber, with or without a belt, is capable of excellent accuracy. If everything is kept equal (which would be nearly impossible) a beltless round might have a very minuscule (think in terms of less than a tenth of a minute of angle) better accuracy potential, but even that is suspect. That the military has used and still uses the 300WinMag for sniper use confirms for me the idea is without much merit.
For all belted rounds, headspace occurs at the belt, technically. But that's for folks using factory ammo. For handloaders, we can adjust the sizing dies (using neck dies, shoulder bump dies, and FL size dies) to move the shoulder back .001-.002" from fired case measurements, creating a shoulder headspacing situation. At that point, in my specific rifle, with my specific handloads, headspacong occurs at the shoulder, not the belt.