I will definitely do a complete bucket change after the first week in DWC... I find the clay pebbles, as I top water during that week, put a lot of sand in the bucket even though I've thoroughly rinsed them before using. Thereafter, it becomes a judgement call - if I have to raise the EC (or lower it), I might withdraw 1 gallon of water and fiddle with that and put it back in... or if the pH is not staying stable I'll do the same thing... Plants don't usually end up drinking a whole lot until they get to pre-flower/flower stage so just adding a bit of adjusted nutes to top off is generally fine but EC requirements change week to week and I often find it easier just to do a complete bucket change. When they start going through the water heavily, I will then definitely start changing out the entire bucket once a week... ALWAYS clean your bucket or reservoir out with a good dose of hydrogen peroxide to ensure the bucket is clean and free of anything harmful...
Good luck!