Stumbled upon something worth mentioning on this subject: brass is soft and changes... duh. So, depending on how many loadings/firings one has on their brass, or if one switches to a different bullet, one's bushing size they probably want to use can change...again, duh.
The reason I mention this is I'm currently using some brass, that without even realizing it, I've now got 9 firings on, and things have changed. I guess I've gotten better over time at taking care of my brass, because I'm just using run-o'-the-mill Hornady brass and I don't think I've ever had this many firings on these cases before... and they're still good, I'm not running very hot loads so I'm not even really getting loose primer pockets yet or anything... I also changed bullets (which I never really took into consideration).
For 6mm Creedmoor, my "old" loaded-round neck OD was .273" with 108ELD-M's (this was probably true for the first several firings or so I'm guessing), but I hadn't re-measured it in a while because my ammo has been great. Now, after a bunch of loadings/firings, and having switched to 115DTAC's, my "new" loaded-round neck OD is .271"... So, unknowingly, I haven't really been sticking to my formula of "loaded-round neck OD - .004 for bushing size"... I was using a .269" bushing, but I'm going to change to a .267" bushing.
Has this been affecting my neck tension? Probably. Does it matter? IDK. Again, my ammo has been great, so this is definitely "into the weeds" kind of thing, but just something to pay attention to as one's brass neck thickness thins out over multiple firings and/or if one switches bullets...