I don't grow in hydro but I can tell you warmer temperatures promote mold, mildew, and algae as well as containing less dissolved oxygen.
In nature most organisms start becoming active when water temps reach about 60-65f.
Armed with that bit of information if you're running organics you probably need to keep it around 65 at the least. If your running chemical nutes you could possibly get by with lower. Otherwise the cooler the better and room temperature should be fine if you keep light out of the tank. Also the bigger your res the more stable the temperature will be.