I use Pellon lightweight. You want something to keep the fabric from stretching, but I think you also don't want something that's going to add on more bulk than necessary since t-shirt cotton is thicker than quilting cotton. A t-shirt quilt is already going to be a little heavier than your average quilt without thicker Pellon---plus, thicker will make it stiffer, I think---and a lightweight is going to be easier to sew through whether you're quilting or hand tying the quilt.