I check and square every step along the way, and after it's quilted, before the binding, I use a large square ruler and double check that the sides are reasonably the same as their opposites and corners are square. But sometimes they're a little out of whack after the quilting, and if trimming it would mess up the design, I opt for it not to be perfectly squared. I like wide borders because there's more leeway for the final squaring.