I prefer to leave a 1/4" (~1/2cm for the rest of the world) border, then hinge mount and place an overmat that just covers the border to help keep the print flat and away from the glass. Partly because of the old argument that leaving a border gives a barrier through which crud has to diffuse before finding the edges of your image, and partly because I still have the bad habit of composing to the edge of the frame, and was tired of losing part of the image to the mat.

Now if I could just spend less time obsessing about he overmat color for color pictures.

Follow up question: Glass or Acrylic?