Cheap office store guillotine paper cutters tend to tug the paper out of square, especially since they dull rather easily. Dahle even makes guillotines suitable for routine precise cutting of metal shim stock, or entire stacks of paper. No, you don't either need or want one of those in a photo lab; but their intermediate series is excellent for application with the lights on - that way you can easily count your fingers both before and after.
Mat cutters do poorly on thin material. And their squaring arms don't support thin paper well. But if ya just gotta go that route, use a thinner blade than mats themselves call for, and make sure it's still very sharp. Hint : domestic s.e. razor blades hold up ten times better than the cheap import hardware and paint store variety.
And other than the fact that cheapo varieties of rotary trimmers also exist, I can't improve upon what's already been stated : There's Rotatrim, and then there's everything else. I have both a 24 inch one, and a 48 inch one mounted to a big laminated table replete with squaring edge, roll paper holder, and adjustable inset length stops. Neither has ever needed to be resharpened yet.
