If you don't square as you go along, any unevenness in seams gets magnified with each added round. Something that could be trimmed and squared to be unnoticable becomes a mistake that your eye will always be drawn to.
If you only want to square your blocks in the end, I would at least measure them halfway through to see how accurate the measuring and sewing was, and then decide whether adjustments need to be made. Some layouts (barn raising) are great for hiding mistakes, so there are other options. Log cabins are one of the most forgiving quilts, great for beginners and oldtimers alike.