Yahoo! Finance presents an article discussing when it is safe to give out your SSN, and when you should give more thought to doing so. From the article:
Just because someone asks for it doesn’t mean you have to comply, says Michael J. Arata, the author of “Identity Theft For Dummies,” especially since there are only a handful of organizations that actually have a valid need for it. For instance, anytime you’re applying for credit — for a new credit card, a loan, new telephone or cellular service — the creditor will need your Social Security number to run a credit check. You’ll also need to provide it if you are applying for federal or local government benefits such as Social Security, Medicare or Medicaid, unemployment insurance or disability. Another example: If you or your children receive services or aid at the state or local level, such as free or reduced fee lunch or financial aid. The local motor vehicle department, thanks to the USA PATRIOT Act, has the legal right to ask for Social Security numbers, too. In addition, when you complete a cash transaction totaling more than $10,000 you’ll be required to provide your number so that transaction can be reported to the Internal Revenue Service, says ITRC’s Foley.
The article contains a nice chart that divides organizations who request your card into “mandatory” and “optional” groups. It also has a sidebar that tells you what the sections in your social security number mean.