I encountered it in Malaysia, but it was more the taxi drivers asking if you wanted to have a go, they would be working for pimps.
I never did it because of the likely hood of it actually being dangerous. The thought of going to some seedy part of town, i had never been to or seen...too inherently dangerous imho.
I dont know if this would be the case in Japan though.
At the end of the day it is up to you. But i think sex is better when it isnt paid for
