Regarding trimming.
This is a very important operation for pistol brass. In many brands of brass , case length can vary significantly. The result is that when a bullet is seated in the case and crimped, the variable length of the cases will determine the amount, or "hardness" of the crimp being applied. Additionally, it will also determine where in the bullet’s canalure the crimp takes place (low, middle, high, or maybe even out of the cannalure). The amount of crimp and placement of the crimp, in turn, can affect the consistency of the powder’s ignition, and cause velocity changes from shot to shot. Bottom line - we want all the cases to be the same length to avoid these variations.
This is important. IMHO.