I have been sewing since I was 10 (more than 40 years now). I figure the T-shirt wasn't backed when it was made and it isn't any more stretchy now than it was then. Since all my pieces are cut in increments of 4 inches (plus seam allowance) it is easy to 'ease in' when sewing because you know the dimensions/proportions of all the pieces. I would think it would be harder if every piece was just cut some random size - since they were never designed to fit together.