These RPM ranges you're describing are normal. Startup is going to be between 800-950. It's higher because the computer is sensing engine temp and is running those RPMS to help bring oil/fluid temp up to temp.
After driving the car and coming to a stop light the motor is obviously warmer now, so the higher RPMS are no longer needed, so it runs about 500 - 600. I wondered about why it ran so low, as well, when I first noticed.
There is some slight shake at both startup and at stoplights, but it's minimal.
As for changing the timing, I read somewhere that the computer works with the position sensors of the cam to automatically adjust engine timing, so I would think it'd be kinda pointless to mess with it if it resets itself.
Just my 7.62 worth of info.