Testing the ph of distilled/ro water is going to get boring after a while. xD
I would say anyone not fully organic should ph initially, especially if using new or unfamiliar nutes, just to get a baseline. Synthetic nutes can be alkaline or acidic. Water supplies vary from place to place, etc. I wouldn't obsess over it like the hydro guys do
but I think even soil growers need to know what they're putting on their plants, at least out of the gate.
I was having problems with rain water and a liquid MG product that all the other plants and flowers loved, but the cannabis wasn't having any of it. Turns out this MG product was alkaline(~9) out of the bottle and it was causing severe lockouts. Adding some cider vinegar to the mix solved the problem. I'm saving the rest of it for the plants that like it as is, but had I not checked the ph I think the plants would have eventually died. The problem wasn't the ferts, it was my fault for assuming it would be acidic.
Ph testing can be a useful troubleshooting tool for soil growers but otherwise it's unnecessary,imo.