You want the grain to run from fade to fade and the groth ring orientation to be from back to front as best you can get it for solid risers. I have only seen laminated risers with the glue lines running from back to front and because it is essentially plywood the grain is random. I have seen some I beam risers with the groth rings side to side for cosmetics. It would depend mostly on the poundage of the bow, the handle shape and and the corners of the shelf as to whether turning the groth ring orientation side to side will cause harm. I would err on the side of caution because of the time it takes to build these things.
In short, I would always have the grain run fade to fade and definately laminate the riser with glass or phenolic if the groth rings have to be oriented side to side too much. Quarter sawn lumber would be OK as long as the corner of your shelf where it meets the handle doesn't fall smack dab in the middle of a groth ring. Sand the corners off (inside and outside corners), sharp corners cause concentration of stress in those areas. Type of wood makes a difference too. Walnut risers will crack easier than most for example.