You can refer to the street and candid section in this forum to check out. Yes u can ask them to pose for you. Depends on the aims you are getting to achieve. I never ask them for posing before, i love to take them unaware, spontaneous.
Japanese are generally fine the last i went Hokkaido. But there will definitely be folks who gets irritated
Japanese are very friendly.. at least those in Hokkaido. Once I was trying to capture a group of old workers chopping wood naturally, I was caught by them and.... they all posed for me.