I neck size until they don't fit the chamber comfortably any longer. Then I FS them. That's about 5 to7 firings in between full sizing. I find they don't usually need to be trimmed to length while neck sizing only. I think the brass swells a bit & actually comes up a bit shorter. Then, when it's FS'ed, it gets a wee bit longer from getting pushed back into shape in the die.
After you fire your ammo, check your length. I think you'll find they're either the same length or maybe even a tiny bit shorter.
I think it depends on how tight or sloppy your chamber is.

Have fun

Frank in Fla