Except that the stateful packet filter that sometimes comes with a NAT gateway (and that provides the security that people like) depends exactly not at all on NAT and the pain that comes with that kind of address class distinction.
Well I don't have stats but from personal experience (perhaps I have a bit more experience with french rather than US providers), home routers do not use a specific stateful firewall, and use masquerading NAT. So it would depend on the way NAT is done.