It's best to look at the face of a board, rather than the edge. Find a board with the grain running the full length of the board, with as litlle grain run off as possible. What the grain does on the edge of the board is not as important. Remember, this is for a board bow, and I almost always back the board with something... hickory, bamboo, maple, rawhide, silk, cloth, etc.
Check out George's website, he has awesome info on selecting wood and making board bows.