As a matter of fact, my router password and username are both admin.
Mine as well, but I've turned off logging in to the router even if you are connected to the WiFi. The only way to log in to the router is via a wired ethernet connection, and if an attacker has managed to gain physical access to my router, than I've got much bigger problems to worry about.
If you are really concerned about your router, make sure you have a strong WiFi password, you are using WPA2 (and not WEP), set a strong (and different) admin password, and make sure remote log in/remote management/remote access/whatever it's called on your device is turned off.