Brownknows, it makes no difference which way the grain goes as the stress is at a 45 degree angle to the span and switches 90 degrees as you change from positive G's to negative G's. If you don't believe this just check out any strength and materials book on beam design. It is usually easier to put them in with the grain vertical and will work just fine in that manner and is a good compromise; however if the grain ran spanwise they will be just as strong when it comes to resisting G loads.