Some folks do, lots of folk's don't.
Really depends what you're trying to do. If you're going for the best accuracy you can squeeze out of the gun for really long-range shots, then you need the most consistent ammo. Trimming will help you get a very consistent crimp.
If you're loading for shots inside 50 yds, maybe inside 100 yds, it's probably not going to make much difference. But you could do your own tests easily enough to see for yourself if it makes a difference. You'd want to chrono the loads to see if there is more velocity difference, of course, but all that really matters is how they group.
Of course, depending on your gun's inherant accuracy, there may be other factors which would cause greater accuracy loss than trimmed-vs.-untrimmed brass. If I had a tuned, line-bored, super accurate revolver, capable of rifle accuracy, I'd probably take the time to be super precise with my loading to take advantage of the accuracy potential I'd paid for.
Loading on my progressive, the cases come out of the tumbler, into the press, and come out as loaded ammo. For my gun and my purposes, trimming handgun brass is a waste of time.
For a long range shooter, and probably bullseye competetors, it might prove to be useful.
-Sam