Face grain and edge grain bond well when glued together, but end grain glued to anything lacks holding power. So don't waste glue (or precious open time) on end grain if you don't need to. For example, on this mortise-and-tenon joint, apply glue to the faces and edges of the tenon and mating mortise walls, but don't bother with the shoulders—it will just cause extra squeeze-out you'll have to clean up.—from the WOOD® shop