When I first began loading for the 9mm Luger round, I set ten once fired cases aside, numbered them, and measured the length with an electronic micrometer that reads to four decimal places. Then I resized them and prepared them for reloading. Measured them again, after which I loaded and fired them. Measured the fired cases again, resized and measured again, and again fired them.
Repeated this process through ten firings. Bottom line:
After ten repetitions of the above, not one round was outside SAAMI length limits. Beginning then, I quit worrying about case length in straight wall pistol brass. Now and then I check one and have never felt any need to trim. Incidentally, there was a slight amount of shortening when the cases were fired and a slight amount of lengthening during sizing.