We do this on post top tenons when square ruling. If the post is 1/4" wider than called for, we trim the tenon back 1/4" to the correct measurement. Secondarily, it hides the mortise when the post shrinks (as Gabel mentioned). We don't do it on girts, which are usually housed - we'll cut back the entire end, not just the tenon, and use a gradual reduction, not a square cut. CB.
