I'm on the JET Program now, and only one of the hundred or so JETs I know has a TEFL qualification. Around a quarter have teaching or linguistics degrees. Historically JET values an outgoing personality over teaching experience or knowledge. That is changing recently--this year JET began offered a grant to current participants to take online TEFL courses--but I still don't think a TEFL qualification will help you as much as stressing community involvement, interest in Japan, and your tutoring experience. They required a minimum 100-hour program to get the grant, so if you're going to get one, get that.