I find I am more careful about squaring if I consider the project as "special". Right now I am sewing a pre-cut kit where parts of it is missing and I don't seem to care about squaring anything. It's mostly HST's that had already been cut as triangles. I do still iron everything carefully, and ironically, after I checked the center medallion's first four rounds, I am within 1/4" for every side at 36 1/4". I did trim one small edge of an HST that bowed out.
When I do square I do it by centering the 45 degree on the center of the block with whatever square ruler I am using. I have all sizes. My favorite is my 6.5". Next is the 12.5", although the 8.5 & the 10.5 do get used every once in a while. I trim any and all sides that are oversized. There are fun times when I get to rip a block back up and make those seams smaller to bring it up to size. I fudge one way or the other if I have to.