If you are going to machine-quilt, then quilt in the ditch between 2 rows from one side to the other, then turn the quilt one-quarter turn and stitch from one side to the other, then another quarter-turn, and so on. In other words, do not stitch the quilt all in the same direction, but alternate so that half of it is stitched from top to bottom and the other half from bottom to top, half from left to right and then from right to left.
When you begin the diagonal quilting, do the same thing, alternating directions. This practice keeps the quilt from pulling on leaning to one side or the other.
The diagonal quilting will add an important design element to your quilt, as it will add a different dimension from the up-and-down of the squares.