IMO no need to be on edge. Myself and many others have been running pure RO very little to no dKH tanks for years. There are loads of myths out there that still get repeated over an over. Most times it's from people who have never run a low dKH tank planted tank.
My fully degassed pH is 6.25 and I drop pH via CO2 injection to 4.85 on a daily basis. The readings are always very stable. My Rainbowfish show no signs of distress. However if I drop any much further they do begin to get lethargic.
That is the same at any dKH. There is always a limit where fish will show distress. It varies a bit from tank to tank but I believe that has more to do with oxygen levels. CO2 and O2 are not mutually exclusive. The higher the O2 concentration the higher the CO2 concentration can be without affecting livestock.
The people I know who run tanks in this fashion do not experience a pH crashes. They have no problem with nitrifying bacteria dying. Fish do not die from osmotic shock.
And it's not to say that pH crashes are not real. They are almost always tied to very poor maintenance. A well run clean tank at zero dKH has nothing to worry about.
Here's an article from the 2 hr Aquarist site regarding low pH tanks. My tank is pictured from several years back when I was running 1 dKH. Shortly after that article I stopped dosing any carbonates at all and not a thing changed other than the degassed pH level.
Is low pH a concern in planted aquariums? Does the low pH caused by CO2 injection or aquasoil affect livestock or bacteria in the planted aquarium? Should I use buffers to raise the pH? This article examines why pH drops in aquasoil tanks and what should be done (or not done).