I always attach an earth to chassis, but not the audio earth. Remember that the audio earth is both a reference point and a signal return, and the chassis is for safety earth and screening.
I take the incoming mains earth to the chassis directly from the mains connector. To this ground point I also take shields and screens.
From that chassis ground pont I lead out a low value resistor to the main audio reference point which is normally the star point between the smoothing capacitors. All audio circuitry is then referenced to this star point.
These comments apply to line level equipment (preamplifiers, etc.). I have never designed a guitar amp, but would apply the same principles.