I went like 15 years without even knowing about note centers. I didn't really understand what it meant to be in the key of C as opposed to A minor. I knew both of them had the same notes, but I didn't know how to tell which was which.
I've been playin in bands for 30 years. If the root of the chorus is A, I say 'it's in the key of A', although it may or may not be true.
There are key changes. It's quite common to switch between the relative majors and minors such as Crazy Train going from F# minor to A. I Alone by the rock band Live switches between G and E minor for the whole song. The interesting thing is that the bridge section is nothing but a B chord that is in 5th position coming out of the E minor chorus. I call that a floating or dependent section since it simply can't be done without the existence of another section. Well, maybe there's a way to accomplish it using melody that I'm not aware of.
The song Invaders by Iron Maiden has some crazy key changing. The chorus is a shift of a semitone or a tritone. I can't remember, but it's a huge, huge shift to a key very distant from what they're leaving behind.
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