I've learned to double check, then check again, the backing when sandwiching a quilt -- (I'm now using spray baste or Elmer's) because recently I got a pieced picture frame style backing on crooked and then quilted it that way. So I have a quilt with a back that will forever and always be crooked....and will forever haunt me.
Like cutting lumber: measure twice, cut once. Or in this case, check as many times as it takes to make sure your backing is straight and lined up with your quilt top!