I'm on my first grow with a PH meter.
I've had a couple of successful harvests without it - especially outdoors where I still don't monitor PH.
But it's sure been a help, I had yellow top leaves and growth tips just as my PH meter, PH up and PH down arrived.
I was able to recognize my tap water comes out at about 7.5 PH, and that was causing iron lockout
All better now.
My tap water has consistently been about 7.5, so I'm starting to assume that as a default and add 2 drops of PH down without bothering to measure PH (when plain watering).