Are you doing Stitch-in-the-ditch or free motion? If SID, then I start in the middle of the quilt at the edge of a seam line. I have only ever quilted with the top side UP.
When doing FMQ, then I start somewhere close to the center of the quilt - again, with the top side UP.
I can imagine quilting the back side up when the backing fabric has a motiv that is used as the quilting outline.