heh, for some reason when you wrote ring on the pinky finger I thought of gangsters. would be kinda funny if it had anything to do with lesbian gangsters :-)
however to answer your question (or actually to not answer your question), don't know. never realized that the rings were on the pinky finger. maybe it's a personal trait of the manga-ka. if you pay attention to fashion, have you noticed any models wearing pinky rings as accessories?