score:4
Since Official Site of Korea Tourism Org.: Alien Registration:
A Korean embassy or consulate can issue two types of visas: a short-term visa for visitors who want to stay up to 90 days and a special long-term visa for periods longer than 90 days. A visitor with a special long-term visa is required to apply for alien registration at a local immigration office within 90 days of arrival.
I think one can work backwards and deduce that for stays under 90 days (standard visa) such is not required.