Generally speaking, I like to see plywood veneer grain running in the direction of the longest dimension. It just looks more natural to the eye. However, there may be reasons for not following this simple rule of thumb. If running the grain in the direction of the longest dimension causes a lot of waste of the material, then you may want to consider a more economical orientation. There may be features of the grain that you want to highlight in the pattern, which may cause you to orient the direction a particular way. IMHO, it's mostly a visual thing. Strength of the plywood doesn't really come into play, unless you are doing construction or building cabinets/furniture.
