I started trimming .38 and .357 brass because it bugged the crap out of me that my crimps were not consistent. I fall under the category of shooters that isn't good enough to tell the difference though.
One thing I can tell you is, that if you trim all of your .38 brass to the same length, it is very unlikely that you will ever have to trim it again. I know I have reloaded some of mine 4 or 5 times and they are still within a couple of thousandths of when I first sized them. It is a different story on my .357 brass with heavier loads.