I heavily starch flannel before cutting. Even though you were careful, it sounds as if the fabric distorted with handling.
Do you mean you didn't add a border; just that the edges of the pieced top are rippled? If that is the case, I would try spray starch blocking the edges to size using Sharon Schamber's method. She demonstrates it on a block on Youtube. It would be more awkward doing an entire border at a time, but it could be done.
If the rippling was from actually adding a border, then I would do as the previous poster mentioned. Since you mentioned sashing, I'm thinking you have sashing strips all around the edges of the quilt? If so, that would be the way to go -- take off the outer longer sashing strips, measure the quilt through the middle, then cut the sashing strips to match the middle measurement. What you will end up doing is easing the excessive material in the top to the accurate measurement of the sashing strip. For extra security, I would recommend heavily starching the sashing strip before cutting to size so its size remains stable while you are pinning and sewing.