The less you physically attach the pieces together, the better, because the photo, the mat, and the backing all expand and contract individually with variations in temperature, so you want them to be fairly "loosely" attached to prevent warping. I use two small pieces of tape to attach the photo to the top of the mat, and then a small strip of tape which joins the top of the mat to the top of the backing board, like a hinge. This allows everything to move around as necessary inside a frame, as the temperature and humidity fluctuate.
Regards,
Paul
http://www.timpaphotography.com/
http://www.timpaphotography.com/blog/