I've got a Fiskars that doesn't cut straight or make square corners. Still, I use it for quick trims that I can line up on something other than the trimmer. I also have a real dinosaur of a guillotine cutter (reject from an old photographer) that cuts perfectly but is too heavy to haul around. The best solution I've found (until I find a better trimmer) is a quilting ruler and rotary cutter -- and very careful attention.
Bridget