Marrowgar will always bone spike. The only thing worth noting is that he won't bone spike his main target. On heroic, however, he does not target anything during his bone storm and, as such, everyone is an eligible target.
That said, if we're talking strictly normal or pre-bone storm heroic, then, yeah, standing in front of the boss MIGHT make a difference. I don't know that Marrowgar actually has a cleave, but he definitely drops fire in front of him, which may serve the same purpose.