Good luck taking it back to Joanns!
The last time I bought some backing from Joanns I had a heck of a time getting it to square up on the quilting machine. I seemed like it kept stretching!!!! I called that my quilt from He$$. I prewashed my fabric and didn't notice that it shrank any....
I'd say that if it didn't shrink anymore when you pre-washed, it shouldn't after you're done.
good luck!