FWIW, my '05 STi completely stock was pinging on occasion on CA 91 octane. It was particularly bad after a reset. It would ping all over the place while trying to learn. It concerned me enough that when I had my car tuned, the tuner and I paid particular attention to minor detonation events and tuned them out. May have lost a few HP, but the car never pings and makes consistent HP.
One thing that surprises me is how many '04 STi owners I've met that refuse to take in their cars for the reflash fearing power reduction. This while their cars are pinging like CRAZY on pump gas. If it were me, I'd take the reflash and just put in a 94 mix to assure safe power. But that's just me.