Squaring up begins as soon as you sew two pieces of a block together. Everytime another piece is added, the section is squared. When the entire block is pieced, the block is squared again. After adding sashing or borders, I stay stitch around the quilt top, so the fabric doesn't stretch when handling, and adding other blocks, sashing, etc.
By squaring up as you piece the quilt top, it will be square when the top is finished.