It depends on what kind of coverage you feel that you may need.
Loss Damage Waiver when purchased, relieves you of financial responsibility for any loss of or damage to the rental car itself (as long as you are complying with the terms of your rental agreement)
SLI or Supplemental Liability Insurance offers additional coverage for public liability or property damage (to other people, property and vehicles but NOT the rental car) beyond what is the minimum required by state law (which may already be included in your rate)
Personal Accident and Effects protection offer coverage for medical expenses, and coverage for your belongings that are in the rental car if stolen.
You may already have all or some coverages through
1. Your own personal automobile insurance policy
2. The credit card you use
Contact your insurance adjustor and your credit card issuer to determine if you have any coverage (and if so, what the terms, conditions and limits of coverage may be) before you decide if you want to/need to purchase any optional coverage from the rental car company.
Read more about Rental Car Insurance at http://www.best-car-rental-tips.com/rental-car-insurance.html