'Proper' pH depends on the type of system your plants are in.
Roots-out systems, where the roots dry out between waterings, have a natural pH drop during the dry out period. 'Proper' pH range will vary with each system but top and bottom pH (edit: of your nutrient solution) will generally be 5.5 to 6.1.
Roots-In systems, where the roots are constantly exposed to the nutrient system, do not have a 'dry out' induced pH swing. Best results are obtained by setting the pH at 5.4 and letting the pH naturally rise as the plants eat nutrients. Res size determines how fast this swing occurs. 5.4 - 5.8 is where you're going to find the best results with these systems.
Just figured I'd clarify.
I would say it is also strain dependent. Mag hungry OG's don't like 5.6(Mag valley)
I have run 5.2-5.5 and 5.8-6.0 with OG's and they perform the same.
If you do run 5.2 I suggest letting it drift up to 6.0 periodically.
Staying at any specific pH is not beneficial. As growers find out using dosers for pH control.
Lastly the higher the TDS the less pH matters. Real easy to kill plants under 600ppm if pH is off.