I did both for awhile. Water changes were with RO and then if I needed to fill the tank due to water loss, Id just refil with tap. I usually loose 5 gallons of water a week to evaporation.
I did go get a filter system for my tap, and used it for awhile, but my rays seem to do good now with just tap water. Our water is a little hard here, but for the past 8 months Ive just used tap and they are fine. Prime helps with the water..