I do it either way, depending on the quilt and what's on the back and how it looks with the front. I do like to double the folded back binding, though, to help with the wear factor.
I just mended my sis's quilt, (over 30 years old [the quilt I mean])the backing had been folded over the front and was getting a little worn on the edge, so I just added a regular binding over the folded over binding. Machine stitched it to the back, folded it to the front, turned back the raw edge a generous half inch, then machine stitched it to the front. It gets a lot of use and I wanted the sturdier stitch on both sides.