It really depends on the deck (and it's a matter of personal taste). There are decks where the art is very self-contained, like Robert Place's, and they look great with borders around them. A little "air" around each motif.
Other decks have such an interesting design that without borders, interactions between the cards develop which you couldn't see before. The Thoth is an example for that - the use of projective geometry becomes clear when the deck is trimmed. (Of course, you have to know the information printed on the frames but that's no problem, it's the most basic information anyway, and keywords are for me better kept in my head than in reading before me - it would frame my interpretation....)
Another kind of deck has such a strong narrative pull that they look best borderless. The Silhouettes is a point in case; the 2nd edition is borderless and in readings, the deck really starts to move - before you notice it, the little figures start to jump from card to card ;-)
Then there are decks where the borders are simply very, very ugly and you ask yourself, why didn't they choose a nice, unobtrusive border? Spiral and Celestial have suffered from that treatment. The weird characterless colour of the Haindl's frames really upset me, it didn't harmonize at all with the cards and I cut it off.
The Lo Scarabeo has really horrid overdone frames - some knotwork thrown in for good measure - after I trimmed it, it looked really nice and fresh and the water colours could breathe.
The ugly, asymmetric green borders on the Vacchetta with the multilingual titles - ugh. I felt this deck was really happy when I liberated it!
But in other decks, I feel the frames add something - Kat Black's decks, the Shadowscapes (lovery colour choice and perfect proportions), Anna K.
I've seen that others trimmed them but I didn't. I really like those borders. They add something. Most of my decks are un-trimmed but I can't promise anything for the future ;-) But I love giving them a nice edge treatment.
I could have lived with the white frames of the Cosmic and Renaissance, but trimming and edging set them free for me. And the bleak Night Sun looks much better without the visual relief that borders give. I trimmed it and edged it in black - now it really looks sombre indeed!
The Commemorative looks nicer to me trimmed and edged, others would hate to do that to their precious deck.
Isn't it great that we're all different?