There can be no harm is buying the additional insurance offered by the rental car company, but it may not be necessary, and it is costly. The best idea is to call your insurance agent and ask if your regular car insurance covers you. Also, rental insurance may be included for free if you rent the car with your credit card. Once you get to the car rental booth at the airport, it’s too late to ask these people, so the smart thing to do is to make a few phone calls ahead of time to find out if you’re already covered. You should have at least the same coverage for a rental car that you have on your regular car.
The rules governing rental car insurance vary from state to state and nation to nation. Before renting a car in a foreign country make double sure you are insured. Getting in an accident in a foreign country without insurance can cause you untold grief. It’s best to make sure you are covered.
Just to protect yourself, take some pictures of your rental car when you get it, making note of any dents, etc.