For the heck of it, I thought I'd give a shot at trimming a card myself. I've never attempted to trim a card other than my own customs or occasionally a cut-out oddball from a magazine or box, but for the sake of an interesting blog post, I'll give it a try.
Here's a 1990 Score Tony Gwynn card I found among my many Gwynn dupes (He's my all-time favorite player, so I've got a lot.) You can see this card's top corners are both clearly less than gem mint. Let's see if I can take the ol' paper cutter to it to improve them.
Well, I doubt this card would grade out to a 10 after going under the knife-- a bit off-center to the right, and the corners still aren't perfect-- but at a quick glance, I'd say those top corners are improved. If this was hypothetically a valuable card like a 2011 Topps Update Mike Trout or a 1986-87 Fleer Michael Jordan, the alteration may have helped bump up the grade by a few hundred dollars' worth, assuming the grading entity didn't take any measurements into account and just gave it a rough once-over.
So there you have it. And please note that this is my first time trimming a real card and that I am doing this at 1:30 in the morning while having trouble sleeping. I'd imaging that someone who does this a lot (and on a full night's sleep) would do a better job.
So yeah, it's a bit scary if you're concerned about things such as altered cards and/or are someone who pays a premium for slabbed 10's.
Lest anyone think I was contaminating the hobby with altered '90 Score Gwynns, I pulled the ol' Upper Deck X treatment on this guy, with some artificial "die cuts" to make sure it never gets mistaken for an unaltered card. (Sorry, Tony-- no disrespect intended! Thank you for your sacrifice.)