Can I turn off my Guest account and still use Find My Mac?
Yes, it will still function without the guest account.  However, in order for Find my Mac to locate your Mac, the Mac needs to be connected to the internet.  If your account is secure and your Mac is stolen, presumably the thief would not know your password, and so wouldn't be able to login and join a WiFi network.

Leaving guest access enabled allows them to login without a password, in the hopes that they then join a WiFi network and your Mac is located (and immediately locked/wiped/etc).