I would say it is pointless to do so.
Unless a visitor is checking out or logging in, there is no point to differentiating between them and any other visitor. More importantly, there is no logical basis on which to do so.
IP? Nope. Some IP addresses are shared.
Cookie? Well, already holds session for visitors and registered clients alike. Could hold a flag to do so, but again, whats the point? Are you suggesting that the cart should present a query as to whether someone wants to be a guest or not and flag the cookie in response? Why? The fact that they are there and viewing pages, makes them your guest.
David