Walnut will vary considerably in terms of tendency to split along the grain. On quarter sawn walnut you definitely want the grain to run along the wrist, not across it. If you start with a plank with straight grain in it, you need to angle the pattern to achieve this. The tendency when starting with such a plank is to say, "Gee, if I put the pattern on this way I can get two blanks, but if I angle it to get correct flow through the wrist I can only get one." Well, the best thing to do is to cut one good one and not two bad ones!
Tom