If the cashier knows you, then it's not an anonymous purchase. But it doesn't matter, because as mentioned, you can't access MtGox over Tor. You could use public wifi, but if you access it from an IP address linked to your identity, your MtGox account is tainted and you need to mix your coins anyway. That would be the time. Another option I forgot to mention is that some people in this community, such as Captain Mal and Sugar Mama, will trade you bitcoins for cash in mail, Moneypaks and few other payment methods, but they charge as much as 10%.