I usually end up piecing a strip through the middle on twin sized quilts since I am piecing two pieces together anyway. (my selvedges run horizontal so I buy 2 times the width needed and get the length of 40+pieced+40)
However, I just bought 108" fabric for a 102" square quilt and it shrunk after cold water wash and low heat dry. So I had to piece a strip. I wasn't willing to try to cut a seam straight through the middle of 108 (or less now) so I ended up putting it across the top. Why? Well if I ever try to turn the top down, that would be the natural place to have something different. We'll see if it actually looks good after I get it quilted.