I do square up units and then blocks and then quilt block quadrants. They aren't usually too far off. Points are taken into consideration and to leave that elusive 1/4 inch seam allowance for later.
I am thankful to have great instructors at the LQS that taught square-up techniques.
I've been working through a BOM project with careful cutting and piecing only to result in being "off". For now, they will good enough!