That is what I think looks best too. Usually I leave space around the perimeter of the print, so that there is plenty of border. Print size actually becomes something like 13.5 x 18". Having three inches on each side around the mat board I think helps tremendously. The only thing I don't like about 20x24 mat board is that most rag stock for window mats are 32x40 inches, leaving an 8" piece of trim along the entire length that becomes waste.
"Often moments come looking for us". - Robert Frank
"Make good art!" - Neil Gaiman
"...the heart and mind are the true lens of the camera". - Yousuf Karsh