Using .308 as an example, I load new cases and fire once, as is. Same, of course, for factory new ammo.
During the first reloading process, I trim to the minimum length (2.000). I'll then shoot the lot and measure until I have to trim again. When I have to trim again, I'll reload and shoot once afterward, then the case gets scrapped.
I usually average 5-6 reloadings of fired cases after the first trim.
Hiding in plain sight...