Green pass is not worth it. The question is how good is your Japanese?. Your best best is planning out your travel in advance and going to a Travel Centre and asking for a reservation ticket which is free. There are travel centres at most major stations. This guarantee's you a seat.

Try to avoid using un-reserved if you can. You will be VERY lucking to get a seat and I remember coming all the way back from Osaka stood up all the way in to Tokyo...that....HURT x.x.

-mrploddy

PS The Japan Centre last time I bought a pass in '05 added a booking fee on top of the original pass cost circa £15 I think