I think slotted trimmers are the cat's meow for trimming up squares and keeping the patch design intact. Be sure to get one that has the diagonal lines crossed in the middle, so you can line up stitching lines perfectly.
https://quiltcut.com/shop/quiltcut-s...14%C2%BD-x-17/
Even better, I use the ruler right after I press the quilted square and it holds in the heat (and steam if you've used steam) just long enough to make the square perfectly FLAT.
It's a joy sewing together the squares I trim with this ruler as they are so perfectly square and flat.