Roger and some of the other admins here know more about this topic than I do, so I hope more of them chime in. I don’t know about the potential for shoulder injury, but I though jd said it fair when he suggested that plenty of pitchers have done very well for themselves while ‘throwing across the body.’ RBish also appears to have thought alot about this, I believe, so his comments would be welcome, too.
From what I think I understand, as long as the dragline made by your post foot finishes on a line (an imaginary line that is: middle of the rubber-to-middle of HP) that is directed straight to the target, it doesn’t matter whether you throw across your body or not. A dragline that looks the same for every delivery and consistently finishes straight at the target should indicate that you are also releasing the ball from a consistent point, so your control should be very good–regardless of whether your stride foot lands slightly open, straight-to-the-plate, or slightly closed.
If your dragline looks the same from pitch to pitch, but does not finish on a line, middle of rubber-to-middle of HP, then the appropriate adjustment is to move over on the rubber by the same amount of distance as needed to bring the finish of your dragline onto that middle-middle line. [Note, it does not matter if the overall shape of your dragline is an arc, a zig-zag, has a brief gap, or any of that sort of thing–the important indicator for your release point is how the dragline finishes].
If there is any extra stress on the shoulder from throwing across the body then, at the very least, I would think you should pay attention to conditioning and strengthening the decelerator muscle groups on the posterior side of your shoulder–but that is advisable for all pitchers, IMO, not just ones who land slightly closed.
