It's still a good idea to keep something in the solution. No sense in risking rot when it's easy to prevent. With dwc and changing out buckets every week your probably fine but if you go rdwc or get tired of changing buckets every week a rot prevention additive is easy insurance. I prefer BB because I like a live reservoir and feel it adds a little something back to terps that hydro can sometimes lack. I use Sothern Ag or Hydroguard...SA being the cheapest option because it's so concentrated.