It looks like you have already bound your quilt as well, you may want to quilt very slowly to allow the feed dogs to really do their job of moving the top & bottom fabrics all through at the same time. Normally you would quilt first as the quilt sandwich may shift some as you are quilting and cause a pucker near your binding.
It's a beautiful quilt, and looks like the disappearing 4 patch design, I really like the sample quilting you did, and would leave it just like that, rather than put in the diagonal going in the other direction as well.
I would also do the diagonal stitching through the whole quilt, not just an X in the squares as you will need to stop & start alot if you do each individual square separately. JMHO