It will change naturally through the day too, got to be careful of "chasing" the ph
it's a frame of reference, test at lights on (before they start to drink), all other tests during the day are just a frame reference to ph drift. Adjust only with adding water or nutrient solution, eventually you will have an average adjustment to go by and your testing will identify if something is going wrong with uptake that you would correlate with your observation of growth. That is where changing the res gives a "reset" but that can stress the plants because they need the daily ph swing as the different nutrients, minerals, and micros are absorbed at different PHs. Ah, ha! Of course the ladies will read this as stress, all stress retards growth. A good way to look at what your doing is eliminating environmental stress, hydroponics can be a vessel for doing exactly that, that is in fact the premise of using these techniques. In order to eliminate these stresses you have to look at it from the standpoint of what they like or don't like in their natural environment.
When you hear it said that hydro should be at 5.8 ph @ 600-1200 ppm, that is at res change and by change I mean at addition of fluid.