I trimmed my Deviant Moon after my parrot chewed the corner of the Ten of Pentacles.
The major effects were that the deck seems much more alive without the white borders and it's much easier to shuffle, due to the card size being slightly smaller.
I'm thinking about taking a pair of scissors to my pocket RWS, but I'm worried that it would be too small to handle afterward.
I think I would only ever trim a deck with white borders, though. The decks I have with black borders, the borders make the images pop, somehow. White borders detract from the image, IMO