I usually trim after I've fired a few times (though never handguns). I usually buy quite a few pieces of brass that I measure in 3lb coffee cans....I usuallly have 2 3lbs coffee cans full of brass for each rifle. I prep all of it on cold snowey nights and use it thoughout the year. After its all fired, I resize and measure, and then trim all of it at the same time to min length........that way for the rest of the year...and for some calibers...for the rest of my life...I just reach into the appropriate can, prime and load.