Since tshirts tend to stretch, I'd tie the corners too. However, if the tshirt has been stabilized with a iron-on type backing, it may be stable enough to actually stitch right on the tshirts. If the foot of the sewing machine pushes the fabric around, you'll have tie it. I have a foot that "bounces" up and down instead of pushing forward. Don't know if a walking foot would work well on tshirts.