Here's one of those "you should always" things that's actually good advice. You should always sew the borders on with the border fabric on top and the quilt next to the feed dogs. Here's why, unless you're using a machine with an even feed system or a walking foot the fabric on the bottom always feeds ever so slightly more than the top fabric. It "eases" the bottom fabric. If you sew with the border on the bottom it will ease it to the quilt top and make them ruffly.
I never measure. I pin the borders on from the center out and sew with the border on top. I have a Pfaff with dual feed and I still always sew my borders on with the border fabric on top.