Top two: Wisdom of the Oracle
Bottom: Shamanic Medicine Oracle
As you can see, I've got pictures of a deck that I trimmed, which I then edged in red, and one that I only edged (this one in a brick red). Why do I modify my decks? To some every deck gets modified and to others it's sacrilegious to modify your decks, and some have just never heard of such a thing.
I don't trim every deck because I don't have a hatred of borders. For example, one deck that is commonly trimmed (and even the creatrix modified her copy) is the Mary-el Tarot, but I love those glossy deep black borders, they remind me a bit of a scrying mirror, so I've left them on. I don't even notice the unevenness of them most of the time unless I focus on them. I did, however, edge my copy in black because when the fronts and backs of cards are black but the edges are raw and white it just doesn't look right. Edging them with a black marker makes the deck look more professional, more finished, and it also protects the edges from the oils of your hands so that the deck will look better longer and last longer.
That makes my other point of edging most of my decks, is that most have raw paper edges since they're printed in sheets and then cut. Those raw paper edges will turn gray and get frayed from use so using markers or ink pads that are archival ink protects those edges. Because of this I'll even use white to edge my decks that have white borders front and back.
Decks that I choose to trim are usually due either to size, like my large Thoth, or because the borders are too ornate or large and detract from the images, again like the Thoth or like the Druidcraft Tarot (and it's companion oracle decks, the Druid Animal and Druid Plant Oracles). Like many, I have difficulty shuffling really large decks, especially tarot decks, because it hurts my hands--only those with large hands don't have this trouble. And ornate borders distract one from reading the cards, as many of us don't find them "framed" but rather find the border fights with the image--again, the Thoth and Druidcraft are great example of this. Freed of those borders, the images really pop. The same held true for The Wizard's Tarot and trimming that one made for a wonderful pocket-sized deck.
The last reason that I modify my decks is that it's a wonderful way to bond with a deck. I usually trim and/or edge from the back, but when I put them back into a finished stack I look at each one. You can get really creative with coloration, some people do rainbow looks even, and really make a deck uniquely yours. Some of my favorites have come from layering colors.
If you've never thought about modifying your decks but want to give it a whirl, or never heard of such a thing and want to see some great examples, do a search on YouTube where there are some wonderful videos of modified decks and tutorials for how to modify your decks. My favorite tools are Tim Holtz Distressed Ink Mini Pads and Prismacolor chisel tip markers.
Have fun with your decks! Remember, while tarot is spiritual and you might do it professionally, it can also be a lot of fun! There's nothing sacrilegious about creating a closer bond with your decks and protecting them!