[biginjapan]

It depends heavily on your situation. If you're going there to be an English teacher its not required to know any Japanese. If you're going there to travel then you probably won't have enough time to make any friends regardless if you knew it anyway.

Knowing Japanese is never really required, but it will make your experience better. When you only can speak English, you are limited to who you can make friends with. Obviously if you speak Japanese fluently this opens up a lot of doors for making new friends and new opportunities for careers.