I would use the default gateway IP and not the ISP DNS.....if the ISP for some reason altered their DNS IP Address (unlikey but possible) you would run into problems and have to update all your clients. Where as the router (as MLF said) would receive the new IP via DHCP.

Usually when using static IP addresses I almost always still push out DNS settings from a DHCP server to the clients to provide a quick way to update the servers/hosts should an urgent DNS change occur for whatever reason.

Networks have a habbit of grinding to a halt after a while when DNS goes wrong....