when you measure through the center of the quilt, top to bottom you should cut your border strip this measurement...unless you are mitering your corners?
attach the side borders with the border fabric on the bottom and the quilt on top. then measure across the quilt top from side to side through the center and cut your top and bottom borders to this measurement. add them to the top and bottom again with the border strips on the bottom and the quilt on top. pinning is a personal thing, some people do, some don't.