Yes, almost all insurance policies value cars based on ACV. That's much easier to come up with on cars like the Camry, Accord, Taurus, etc... as they're a dime a dozen and don't have any collectible value. Cars like the Viper can be much more difficult to determine the "ACV" because the car is rare, is a collectible and varies so much depending on mileage, condition, etc....
Agreed value policies are generally done by specialty insurance companies. They agree to give you xx,*** if the car is totalled. The premium is cheap. But the policy is restrictive and you MUST abide by it. Some standard auto policies will write an agreed value, but you have to ask.
If you have your Viper on a standard "ACV" policy, then as has been said, just keep all documentation. If the car gets totalled, you may have to do some legwork, but you'll likely wind up with a fair settlement. The insurance company will likely use a service, whether it's CCC or a book like NADA, to come up with a value. If you think it's wrong, you can provide them with proof. Things such as receipts, ads showing other cars similar to yours, etc... If you can document and prove their offer is inaccurate, they will change it and base it on the proof you've provided. Believe me, they'd rather pay you the extra couple thousand dollars than have to defend a case in court where you've got proof they undervalued the car. Not good business practices.
Steve