Dark Cloud 2, is it necessary to get ALL of the medals?

I know that you need the medals to exchange with Mayor Need, but other than that do we really need to get all of the medals? What happens if we don't get all of the medals? I read somewhere that you can't unlock Moon flower palace of something like that? So I was wondering if we really need to get ALL of the medals or not? does it affect us from doing some kind of event if we don't get it all? Please and Thank you!