The two main areas of control that you achieve by trimming, other than complying with spec for function are varying bells on the case mouth when expanding and the slightly different amount of bullet pull that could result from a slight variance in the amount of taper crimp. Oh God! that could involve geometry!
Minisiscule, most likely, but hey, it's not the kill, it's the thrill of the chase! When I buy brass from Starline I trim it to one uniform length. If it's once fired factory, I don't bother with the autoloader brass, because most of what I check with the caliper, is too short already. High performance revolver loads get trimmed for the two reasons above!
Sometimes I wonder why some reloaders even bother to tumble brass which is mostly visual, since they're in such a hurry to shoot. Myself, I like to reload! Then, at a later point, I like to shoot. Two different events at two different times!
