When I make T-Shirt quilts I use fat bat and tie with 3 strands of embroidery floss. I take an actual stitch and then tie it off with a square knot. There is no way I could hand quilt through all those thick layers of fabric when you consider the weight of the jersy knit most T-shirts are made out of plus the fusible interfacing you need to apply to keep them from stretching out of shape and the backing. My little #11 betweens would bend after just a few stitches. I have a hard enough time getting through all those layers just to do the ties. I tie every 3". I think it looks just fine but I too think of the end result as more of a comforter than a quilt. That doesn't stop me from calling it a quilt though!
I am going to try quilt as you go (QAYG) on my next T-shirt quilt. I will use warm and natural batting and free motion quilt to follow the design imprint of the T. I can't wait to get started on that one. Have most of the blocks already cut and prepped with interfacing. Should be an interesting new technique for me. I have not done QAYG yet