Depends on your water. find out your TDS..
I did RO only for a year and it was ok but it was only a year.
Lots of people only use tap water but who knows about their success and wether or not tapwater was to blame for their issues (hair algae).
I would say that it is better to start with as pure water as possible RO DI... to allow you to have better control over your parameters. It's really not that expensive, not when you think about the money and effort you would spend on fixing the problems the water may present.
Think about it
