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My view is that it's worth bearing in mind why the guide may have volunteered his/her time. Sometimes you may have an older volunteer with plenty of free time who wants to keep their language skills brushed up. Other times you may have a younger student anxious to develop their own skills. What you won't be able to count on is a thorough understanding of the places they will be 'guides' for you of. (My experience has been of volunteer guides at museums and places such as Meiji Mura, they had all the good will in the world, but were not particularly knowledgeable even about these places/facilities they were so obviously passionate about.)

My advice is to do a bit of preparatory work yourself. The guides' main reason to volunteer is usually to develop their language skills and engage in cultural exchange. Relying on them for thorough information about the places you want to see and visit can prove disappointing. However, if you combine their services with those provided by the attractions you'd like to visit you can tap into expert opinions.

Most places have Japanese speaking guides with thorough knowledge in their area of interest. You can email the places you want to see to find out the timetables of talks and tours. A volunteer guide accompanying you will have a more manageable task interpreting information for you than having to provide it for themselves. (This certainly works for me - my husband speaks Japanese but doesn't read or write it, but through him I have access to the opinions of experts and scholars, he makes a wonderful volunteer guide!)