No, I don't trim, and I would never do it. I don't really like the look of borderless decks. I'm not a fan of huge big borders that take space from the picture either, but a small narrow border that protects the image itself from being too damaged after a decade of shuffling, is just fine with me.
But if somebody wants to trim their own reading deck, who am I to say they can't? Of course you can. You can cut holes in your clothes too if you like, I don't mind. If you're happy that way and it's your style, go for it. It's your deck, be happy with it.
What makes me a little sad is the thought of people artificially aging and "mutilating" (yes) a deck and then decide it's not for them after all... such unnecessary waste of a perfectly good deck that somebody else might have liked in its original un-trimmed shape.
Personally I don't mind previously used decks at all, and I like things that have been worn in gently over a long time – but I know that will eventually happen anyway if I use my cards long enough, so I take care to keep my decks as nice as I can for as long as I can. I only read with five of them regularly anyway, and I might eventually let some of the others go to new homes, so it makes sense for me to keep them as "new" as possible. The next owner can do whatever they like, but I'll let it be their choice.
With all that said, I have a pocket Rider-Waite that I've bent this way and that, dipped in tea, shuffled a million times and scraped off the fugly machine-font titles from with a knife. It looks like the saddest little brown pretzel and the corners are all fuzzy. It's the only deck I've ever deliberately messed up, and I don't regret doing it at all. I couldn't read with it when it was new, the blues and yellows were too bright and if I was ever going to learn the RWS I had to fix that.