Your quilt turned out great. I have made and quilted many T-Shirt quilts. I would stich in the ditch around each of your Squares following your material design. Then in the square itself, because your quilt has a comtemporty look , I would sew ramdom lines across the blocks. maybe two or three per square. Like start at one corner and sew a line diag. to the other corner. , then about 2" from the top on one side to about three inches on the bottom of the oppesite side. You will find that quilting t-shirt material to much even with stabalizer will make the material pucker. I am sure what ever you decide to do it will be Beautiful.