I machine stitch my binding on before I trim away the backing and batting. The backing seems to lay smoother and doesn't get the puckers in it like it does if it's trimmed. I also pin on the binding before I stitch it on the front. After it's stitched on, I trim away the extra using scissors, leaving about 1/8th inch of the batting beyond the edge of the seem allowance and hand stitch it down in the back. I lost points at a quilt show because my binding was not full so I've done it this way since.