Photo framing: matting offset
David Logan in his excellent book on framing (Mat, Mount and Frame It Yourself) discusses Weighted (where the top and sides are the same and the bottom (or top) is heavier) and Balanced (where the sides are the same and the top/bottom are the same) but from what I got out of it, neither method had a "rule". Mostly it came down to you as an artist, selecting the balance that gives the most pleasing results.
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