I trim all my once fired 223/5.56 brass as I find some of it to be longer than spec. I also trim all 38 Special, 357 Mag and 44 Mag once fired brass. I've seen it vary as much as .010" between the same headstamps. Trimming everything to the same length gives me a more precise crimp, especially on the revolver rounds where I'm using a roll crimp. On my full house 357's and 44's I use a heavy roll crimp and a .010" longer case is enough to cause problems by crushing the case. Once they're all trimmed it will be quite a few reloads before they need to be trimmed again.

I've never trimmed any of my semi auto pistol rounds that get a light taper crimp and don't have any problems. Actually those rounds don't get a true crimp but just removing the flare.