After the block is cut out- I like to check the corners and trim the square if I need to. Then I sometimes check with a ruler to make sure they are square. After you make these for awhile you will be able to tell when you need to trim or not.
The more accurate you are here the more headaches you will avoid. If you don't take time to check your blocks could go crooked even tho you sew a straight seam. then your pieced top will be way out of whack(yes a technical term! :mrgreen: ) and you won't know what to do to fix it.
There are other places too to make sure you are on target but this is the one place that causes most of the problems.
However be careful not to trim too much. Gently trim any crooked corners to make it square.