Diagonal Grain pattern

In a bookmatched top, is there any advantage or disadvantage to grain that runs diagonally as opposed to vertically? By this I mean one side of the top has diagonal grain pointing toward the right and the left side has it diagonal towards the left, so the net effect is it kind of looks like an arrowhead - I hope I am explaining this right. Any thoughts - from what I have read, it seems running the grain vertically is preferred, but I am not sure why. Mike