I saw a nifty method for making t-shirt quilts at the quilt show. They used a piece of iron on interfacing big enough for the whole quilt. They cut the t-shirts and ironed them all to the same interfacing, 2 inches apart. Then they ironed the sashing between the t-shirts. Then you just fold and sew. The main thing you have to do is use a teflon pressing sheet.
The interfacing they had was very soft and printed with a 1 inch grid. The interfacing is Pellon #821.