I just completed a quilt top using a jelly roll for the blocks and muslin for the sashing strips. Since I didn't preshrink the jelly roll fabric my plan was to quilt it with cotton batting then shrink it all together. Unfortunately, I realized after I completed the top that the muslin I used for the sashing had been preshrunk. Now what do I do? I'm afraid if I go ahead and quilt/shrink it, my sashing strips are going to puff up.

Are you planning to machine quilt it? Machine quilting stabilizes the quilt sandwich. I think you will be okay as long as your machine quilting lines aren't too far apart.

Yes - I'm machine quilting it. It's a quick quilt so I was planning on doing a meander pattern over the blocks and sashing.

I really wouldn't expect a problem then.

If your quilting is reasonably close, the sashings won't be able to puff up because the quilting will be holding them down, plus the quilting on the non-preshrunk pieces will keep them from shrinking more than the batting shrinks.

I think you will be fine.