This is probably one of the more misunderstood structural topics. In reallity, the strongest shear webs are those that have the grain at 45 degrees to the wingspan. However, when the stress reverses (flying upside down instead of right side up) this switches 90 degrees, still 45 degrees to the span but 90 degrees from before. On real planes, using ply shear webs, there is a definate requirement to always have the ply grain at 45 degrees to the vertical for this very reason. Any one who has studied beam design can confirm this. Two places you can check are http://www.vishay.com/brands/measure.../sgbt/lclb.htm and http://www.silentflight.net/images/s.../shearwebs.pdf. To make a long story short, vertical grain webbing and horizontal grain webbing are equally strong. Just because it is usually easier to do in models, I put the grain vertically unless I'm using a ply shear web then it is at 45 degrees.