First off DWC and bubbleponics aren't exactly the same thing. Either way, it's 'better' to use RO water vs. tap. I put better in ' 's because I don't know it's a fact that you can grow better bud with RO vs tap... but for most people they like RO more as it has less in it than tap. Meaning if you measure the tds in RO water it's virtually 0... there's nothing in it. So if your plants can handle 600 ppm, all 600 of those ppms can be nutrients designed for growing MJ. If you have tap water that's 150ppm out the tap, you can only add so many nutes before the plants are unhappy. Don't pay too much attention to numbers I'm saying, just trying to paint a picture.
My first grow I went to the store and bought 30 gallon bottles of purified water. LOL.. they were looking at me funny. I ordered a 5'stage RO filter from ebay the next week and used that for a couple years. Worked great and cost about $150 or something.
These days I use tap water because I just can't waste that much water anymore. With RO water a lot more goes down the drain than you get clean water. My filter dumped 3 gallons down the drain for every 1 gallon of clean water. I'd rather use 30 gallons of tap water to fill my rez, than 120 gallons.
Also... it really really depends on what YOUR tap water is like. I live in the mountains and have lovely tap water. If it was really full of gunk or had a horrible pH i'd probably go back to RO.
Hope that helps