I've made many T-Shirt quilts and use an iron on tricot on the back of the shirts. The tricot is so soft and you don't even know it's there. There is a stretch to this tricot so you lay it on the shirt with the stretch going the opposite direction from the stretch of the shirt. I have never had a hard or stiff quilt top doing it this way. Or you could use a serger to put the pcs together. Because they stitch so fast and have such a long foot for some reason it doesn't allow the shirts to stretch. I have never had a problem using either of the above ways. They always come out soft and cuddly.