I'm sorta with what Charles is saying here. I'd elaborate a bit and say that I think neck tension is very important if you are shooting very near the lands, or if you shoot a jam. If you jump a bit, it becomes much less meaningful. Maybe even meaningless.
Now, if you anneal, that makes neck tension much more consistent, but more than that, it sorta cures some troubles that are associated with neck tension problems to begin with. It's like, annealing makes it better and less meaningful all in one fell swoop.
I've used one of the K&M presses. (not on my own ammo fyi). I thought it was very consistent in the way it read, and I don't remember speed affecting how it worked as glbriel did. I only ever did about 20 rounds though, and they weren't even my own. That might vary with heavy or light tension. The ones I did were pretty light so there wasnt' much room for variation anyhow.
Just remember, you're gonna get people who say its very important, and others who say it's totally unimportant. And they're probably both right, talking about their own guns. Might be best to do Charles' test.