Great question with many internet opinions. I will admit right here, right now that I have never sanitized our freshwater system in any way. Why you ask? Because before we go on any trip, we fill the tank with city treated water. I consider filling with treated water to be my treatment. Do you think I should do more? Opinions welcome.
On city water, I don't see the need to do it often, maybe once at the beginning of the camping season.
I guess I will continue to do it every outing, since my trips are usually a couple of weeks apart.