If you know a bit of Japanese all the while you don't understand English as well as its local language, it is definitely helpful as there are some signs on the street, metro, airports, or shops written in Japanese (and don't forget that so many Japanese people can't handle even a pretty basic conversation in English).
However, once you know English, there is little to no added benefits to it. The kind of people who speak Japanese yet don't speak English are few and far between, and even if they are in such rare cases they would never consider you, white, speak Japanese. So you would never know he or she speaks a bit of Japanese until and unless you ask it.
That being said, in the case of Taiwan and Hong Kong, and less ture in China, sometimes you understand sings (no English) scattered in the city with your knowledge of Kanji. For example in China, an elevator for the disabled people is tagged as
无障碍电梯. While Japanese people don't understand
电梯, they understand
障碍, which means
disabled. And given the context it is in, they can easily recognize that the elevator is for such people.
And last but not least, some Korean people might feel frustrated to see you, non-Asian, speak Japanese but don't speak even a basic Korean. I would like to encourage you to not talk to them in Japanese. If you still want to use Japanese, it's better to ask something like "Do you speak any other languages such as Japanese?", instead of asking it directly.