I just came back backpacking there for 8 days, i only play around at Seoul with no prior reservation else then the first 2 nights. As suggested you can access everywhere and save the hassle and time to look around for accommodation but i really think that is subjective.

I love nightlife and clubbing and drinking thus i always change accomodation every 1-2 nights because the train stops at 11.30pm to 12 pm. Taxi is extremely expensive -.-

Best night experience is at hongdae with clubs opening all the way to 6am and youngsters bustling around but it is also hardest to find accomodation, booking early solve this and there is a lot of guesthouses there.
Btw, korean guys love asian girls so dont be surprise you will be swarmed.