When renting a car you want to have FULL insurance which you don't have to pay extra for if:
1) You already have a regular (at least) liability insurance policy for your primary car (assuming you own/drive a car right now). If you do not have regular insurance that you pay monthly for then definitely buy the car rentals liability.
2) Some credit cards such as my
CitiBank VISA offer free CDW (collision damage waiver?) assuming that I charge the purchase of the car rental on my Visa.
Definitely call all your credit card companies and ask them what sort of perks/rewards/protection does their card offer.