For fussy cutting, I've found it helpful to "frame" the ruler or template with 1/4 inch tape and then it's easier to see "what I'm going to get" while I'm cutting the fabric.
I also cut the fabric from the back - I like to keep the square as much "on grain" as possible - and it's usually easier to see the the grain lines/threads from the back/wrong side of the material.
I make a lot of round corners, but there are a few things I am OCD about!