The weakest point on shear webs is the glue joint. It is much easier to make a strong joint by gluing the shear webs on either the back or front of the spars. If you make the perfect glue joint, either method will be equally strong. As to which direction the grain goes, it makes little differance as the maximum sheer forces are always at 45 degrees to the span switching 90 degrees as the bending forces change from up (inside loop or normal 1G flying) to down (inverted or outside loop). The only time vertical grain strength is a best direction is if you try to crush the wing from either the top or bottom. Bottom line, either vertical grain or horizontal grain will be equally effective, do it the easiest way or whatever best utilizes your scrap material.