Sometimes sewing several rows or blocks together in the same direction can also distort a quilt. I had to block one recently and I used a can of spray starch to saturate it, then pinned it to the mattress of my king sized bed. The starch really helped it to keep it's new square form once it was dry and the pins were removed. You need a LOT of pins to do this, sometimes one pin for every half to quarter inch along the edges.