I've actually made pretty good length gains never doing a length exercise in my life. Opinions on this vary, but I'm focusing on one aspect before I start another. I think that if I tried to spread my initiative over both length AND girth, one would suffer. So - I have and will be concentrating solely on girth, and when I reach a certain measurement, I'll train length for a little bit before hitting girth again.
Can it hurt? I've HEARD that stretching in close proximal time to doing a girth workout that you run the risk of dimishing your girth workout. Also, I can't imagine doing a stretch routine after a girth routine - my penis is waaay too tired. I posted a while back about stretching BEFORE a girth workout, which I didn't think was a good idea either.