On a recent trip, I got into a big argument with the person at the airport rental car counter. She told me that I couldn't rent the car if I didn't buy their company's insurance. She won the argument -- but who was right?
The trouble with renting a car at an airport is that you don't have much leverage. There you are, tired from your trip, unwilling to lug your bags another step, and obviously desperate to get your hands on the car and head to your hotel. You are, well, the perfect victim.
There is no need for you to buy the insurance offered by rental car companies unless you have no applicable coverage of your own. But chances are you already do have a separate source of coverage, and maybe even more than one. Have a look at your car insurance or homeowner's insurance policy -- either of these may well cover car rentals (though some may not cover them if your travel was for an employer). If your travel is for an employer, ask that employer about its insurance -- it might cover the car rental.
Another source of coverage might be your own credit card company -- many companies provide insurance coverage if you use their card to pay for the rental. This might be a benefit you didn't even know you had, because the companies typically don't charge extra for it. (And how many of us ever read that little contract booklet?)
It's a good idea to look into these possible coverage sources now, before you find yourself tired and impatient at the airport. Make some photocopies of the appropriate policies or contracts to tuck into your travel bag. Showing these copies to the next rental car clerk should nip any arguments in the bud. (To learn more about rental car insurance, as well as rates, requirements, and your rights when renting a car, see Nolo's article Renting a Car.)