Giving volunteer guides a small gift is fine.
Remember that we are discussing this in advance and have the benefit of gathering information. If you realize that an employee who is presented with an unexpected gift or tip will have neither time nor advice to figure out a suitable response (personally and as their company's representative), it makes it easier to go against our wester customs.

Agree, Lcuy. We have experienced exemplary service from many in Japan including, for example, a concierge in Kyoto who we felt went above and beyond any expectations. Other than expressing our sincere thanks to her verbally, I went to find the manager on duty and told him I wished to leave a verbal recommendation for that member of staff for her good service. Whether or not he recorded it anywhere I have no idea, but I felt that was a better way to thank her than a physical gift, since (as is mentioned above) a gift would not be considered appropriate for her simply doing her paid job, and would potentially be very uncomfortable for her.
I do think there's a world of difference between that example and a volunteer guide, doing something for free, and someone helping you during the process of doing their paid job...