If you're visiting the bigger cities and touristy areas, the people will be used enough to working with foreigners for you to be able to get by fine without much Japanese. It's handy to know some basic things though, so you don't end up having to resort to charades for something like 'where is the toilet', or ordering something which isn't what you think it is. A phrase book is a good idea - even if you're not sure how to say the phrase you can just point at it and communicate that way
