I think both methods are correct.
"top off with water and change entire reservoir routinely/weekly"
or
"top off with solution and monitor ph/ppm"
It truly depends on what you want to do and what is convenient for your setup.
Sometimes its more convenient to change then entire reservoir than to monitor the ph/ppm. (single site usually)
Sometimes its way more convenient to monitor the reservoir and not bother changing it. (multi site usually)
I personally like to wipe down the reservoir. Possibly keep two reservoirs handy if they are trash cans or rubbermaids. So that you can have a clean one ready when you change the res.
Edit: Just realized you were referring to aero system. Go with what tat2ue mentioned, "top off with water, change bi-weekly"