My custom for a long time has been to use 1/4" borders on the sides and top, and a 1/2" border on the bottom like you are considering. To my eye, the pictures look "visually centered" in the frame even though the borders are equal all the way around in contravention to the idea of slightly offset top and bottom borders. Unscientifically, I think the larger bottom border visually "lifts" the print in the frame.