I've turned into a "square up as I go" person. I find that it's easier to do that and have them all the same, before sashing, and end up with a better finished piece. My cutting mat is close to my machine and ironing board, so when I get up to press the last seam in a block, it's easy to swing by the cutting mat and plop it down to check it, and square it up if needed.

I used to not square as I went, and boy it only takes a little off here and a little off there to end up with some fullness in places where squares were just a tad too big, or worse yet, open seams in places where squares were a tiny bit too small and didn't get caught all the way in the seam when sewing.