Life is too short to trim pistol brass.

Trimming 9mm brass would seem necessary, as the cartridge headspaces on the case mouth. Consistent length would also lead to a more consistent crimp. However, you will find many/most handloaders, including myself, do not trim 9mm pistol brass, or any straight wall pistol cases. Pistol brass actually tends to shrink, not grow longer as you see with rifle brass. I have never trimmed a single 9mm case and thousands of rounds of 9mm have left my loading bench in spec and functioned perfectly.

* A small note as someone will undoubtedly bring it up. The 9mm case does have a slight taper to it. In this sense it is not technically a straight wall case, but is treated as such for all intents and purposes.