Dear ChrisD:
I have made at least 25 T-shirt quilts so far & I use my long arm & quilt over the entire shirt. Did you use any type of fusible interfacing on the T-shirts before your sewed them together? Using fusible interfacing prevents the puckering of the T-shirts. I use Pellon cotton fusible interfacing myself & it works out fine. I have seen some T-shirt quilts that people have made not using a interfacing (not on this site) that are lumpy & stretched out which is not what you want to see on a quilt. Good luck & congratulations on your first quilt. Now you are officially hooked on quilting. Mary C - Naperville, IL