What are you trying to accomplish?
A static IP on the PC will keep its LAN (internal network) address the same.
A static IP on the router's LAN segment provides you a default gateway
A static IP on the router's WAN segment gives you the same Internet IP, but usually doesn't work if you have a DHCP provider.
If you're port forwarding and need the same internal host, set it up on the PC. If you need the same public IP, it's usually easier to run some sort of dynamic DNS service that maps a domain name to your public IP every so often. Some providers either give you static addresses (like in enterprise circuits such as T1), or offer a static IP option which is usually a few bucks a month.