The necessity depends on the load and chambering and your purposes. Over time, in all calibers I shoot regularly I have gradually moved over either to purchasing new bulk brass or to buying enough of each lot of loaded ammo to result in a good quantity of once-fired brass for reloading. For rifle, I then sort out the most symmetrical cases for match shooting and use the rest for practice. I keep the two sets (match and practice) sorted into ziplock freezer bags by load history so that when a batch gets to the point it needs to be annealed or trimmed I know to run the whole lot at once. That saves time over setting up to do a few here and there.
For pistol, I can't see any practical difference in the performance of carefully sorted symmetrical brass and all the rest. This is basically because a pistol that holds 2 moa at 50 yards is very good, while a rifle that hold ½ moa at 100 yards is very good. That roughly 4:1 difference in very good guns is the difference that determines whether symmetry effects show clearly. Statistically, they will be present in the pistol group, but the effect on group diameter will be about 16 times smaller, which is why it becomes hard to discern.
Finally, as mentioned, there are a few cartridges out there for which some more serious difference in case capacity exists and that can affect pressure and velocity enough that you wouldn't want mixed headstamps in the loads. 300 Win Mag is the biggest offender. At the low end, TulAmmo cases run 87.5 grains case water overflow capacity. At the high end, Norma runs 95.5 grains case water overflow capacity. That's the range. A full load of H4831 for the Norma case behind a 210-grain SMK, would be highly compressed and produce about 27% higher pressure, 150 fps more velocity and a yard less drop at 1000 yards in the TulAmmo case, according to QuickLOAD. So you don't want to mix those cases up. Remington is close to TulAmmo. Winchester is closer to Norma, so a big difference exists in domestic 300 WM headstamps as well.