j_darling2007, don't get confused about the answer. The topic is about the amplifier's ground connection, not the circuit ground. Of course it is correct to remove the negative battery cable (see link below) to open the circuit and shut off power. Once there is no circuit, there is no power to any appliance (including amplifiers) that are connected to that circuit. With a dead circuit it doesn't matter if the power and ground cables are disconnected from the amplifier, or in what order. But even so, even with the circuit open and no available power, the ground is always connected first to the amplifier and disconected last. That is to say, you always ensure that the ground connection is made before the power is connected.
Response: This has absolutely nothing to do with my original question. But to clarify the point of your statement, the damage caused by removing the amplifier ground is due to the fact that removing the amplifier's power ground does not completely remove what you refer to as the circuit ground. If you remove the power ground only, the amplifier will attempt to complete it's circuit through the RCA cables and will most likely blow out the front end of your amplifier because the input circuitry is not desinged to handle the amount of current common in the power ground. This is the reason there is a power ground to begin with.
To the OP, you asked about the reason to ground the amplifier to chassis and I tried to give you a valid reason. Here's another:
Perry Babin said HERE in "Amplifier Installation Notes":
"If the amplifier's ground is properly connected to the body of the vehicle, it will provide a better return path to the charging system's ground than will a ground wire run back to the battery. This is especially true if the ground strap from the engine block to the chassis is upgraded."
Proper grounding of an amplifier can almost always be achieved through the floor pan, and with best possible results. IF chassis grounding repeatedly fails after trying other nearby locations, you can run the ground wire directly through the car and engine compartment to a suitable grounding location. By that time you will have tried and failed to get adequate ground return using the chassis, and so the next best hope for success is a wire directly to the source.
Response: If this is true there is absolutely no reason to ground directly to source, EVER. This also means there should be no such thing as a ground loop because all chassis parts are always at the exact same potential.
How anyone can say chassis sections that could be bolted together, loose, corroded or anything else short of being welded, can possibly provide better continuity than a heavy gauge, multi-strand copper cable is beyond me.
j_darling2007 mentioned the circuit ground, which the question doesn't pertain to. But speaking of the circuit, if the circuit is live, the amplifier is supplied power via two wires: positive and ground. If the ground wire is long and snakes throughout the vehicle rather than being short at the amplifier's mounting location, there is a much greater chance of unseen damage that can occur to the wire. Wouldn't anyone agree that a 15' wire has, at the very least, the potential of fifteen times greater risk of damage than a 1' wire? And if this wire is severed or otherwise badly damaged while the amplifier is connected to a live circuit, what can likely happen?
Response: If, as you suggest above, all points on the chassis are at the same potential then it is irrelevant how long or short the cable is, or if it gets damaged. That is unless the cable is completely severed. If something significant enough to completely severe a 1/0 gauge cable happens to my car, I doubt that an amplifier or two will be at the top of my list of concerns.