Here is what I do? I use Drop and Vile PH testing I test my source water monthly as I'm on a well and things could change. My source has been 6.5PH for several years, but I still check just in case. I check my mixed nutrient PH a couple of times, every time I get a new batch of ferts just to be sure the manufacturer didn't screw the pooch somehow. I check my res PH anytime I see a reason to, i.e. if something looks funny. Keep in mind, I also do a full change out every week in my standard bubble buckets. A Drop and Vile PH kit is maybe 15 bucks and last me a year or better. IMHO if your growing hydro in hempy's or bubblers you can skip the PH meter unless your source water is puckered, in which case you'll be needing an RO setup.