Presumably a strong corollary to this is that having consistent neck expansion is important. I don't mean dimensionally but rather the elasticity of the brass; how it expands on firing. And hence annealing to a consistent brass hardness should be important.
You're right, and lower tension also means lower variance of it, and annealing does lower tension.
Annealing helps people manage their tension to a standard that they established
with load development.
But it's not automatically what everyone should do to an extreme. Some loads like
more tension.
Annealed condition of necks can be pretty difficult to see with seating forces. Often the annealing process changes friction of neck IDs, and they also lose some spring back. So your downsizing of freshly annealed necks spring back outward less on removal of the sizing die, leaving tighter interference fit to seat bullets into. So the bullets end up expanding necks more, which can measure as a higher seating force, even while tension is actually lower.
You can of course normalize friction with lubricants (I recommend dry), but muzzle velocity is still affected by soft -vs- hard necks (by the tension they provide).
Necks getting harder can also increase seating forces, and one way or the other you're probably detecting a problem as increasing seating forces.
For me, when it becomes a battle to match pre-seating forces (off a mandrel), it's time to dip anneal the batch. That's not a full anneal, but stress relieving. I think it's called 'process annealing'.
Since I always partial neck size with bushings, and my adjustment is length of that sizing, I need pretty much normal spring back from necks -to provide for adjustment within sufficient tension range. It's what I'm load developing with and managing.
