I don't know, it seems to me this "open proxy" thing is just a chimera.
Right now, as I write this, I have my choice of 5 different IPs I could be posting under - seems my neighbours aren't quite as current in keeping their LANs secure as I am. For all I know, one or more of them could be TORs or other open proxies. Sometimes, when my own LAN is acting up, I'll wind up on my alternate LAN - two doors down - without even trying. For that matter, most people working on LANs depend on their system admin to set up the internet connections - and would have no way of knowing that they were accessing the web through an open proxy. And of course there are the signal stealers - endemic here; the hotspot users; those who use publicly accessible internet connections (Internet cafes, libraries, etc); and some cities are already offering internet access just by sitting in the local park. My usual ISP never gives my city of residence; and if it was traced back, it could show up as any one of five cities - and I use one of the biggest ISPs in my country. The IP from which someone edits doesn't tell anything about who they are, where they are, or do anything to provide any kind of unique "fingerprint." I really don't see the point.
Risker