I love the way cross hatching adds a calm effect over a pieced background of squares, regardless whether they are in 9 patch blocks or rows.
Many of the current embroidery & applique style quilts use a miscellany of block sizes, randomly placed, with a background of 2″ charm squares. This design is perfectly suited to a cross hatching quilting design that uses the corners of squares as the stitching guide. The stitching has the effect of breaking up the square and creating a secondary design with the adjacent fabric.
This works just as well on larger squares and is very easy to achieve with any method of quilting whether it is by hand, domestic machine or using a longarm machine.
If you are quilting by hand or domestic machine you can run a strip of masking tape diagonally over the squares from corner to corner, then stitch alongside the tape. If you are using a Longarm machine, use a ruler as a guide but remember that your needle will be stitching a 1/4″ away from the ruler so allow for this when aligning the corners.
With all methods, because you are stitching over the bias of the fabric, your lines of stitching will be stronger if you make smaller zig zag lines through the squares rather than one long line of stitching. eg: