I've done many persentations on identity theft over the past couple of years and love to ask this question of the audience.  Far to many groups, businesses, etc. track our social security number in their records.  Far more than have any real need to track it, or access it.   If we stop people from having the number, then they can't lose it, or have someone gain access to it that should not.  One step we can take to reduce the possibility of identity theft.

So who really needs your social security number.

Private Organizations, this includes public utilities.  They don't have to have your number.  They can ask for it, but you can decline to provide it.  Ask for a supervisor if you are pushed.  They may want to run a credit check, or have it in the event that you fail to pay your bill.  Ask if you can provide another method of proof for the credit check, like your previous business with that company.  And if it is not for a credit check, helping them in the event you don't pay your bill isn't something you need to do.

Lenders.  They need your number or they can not access your credit report.  They also need it to issue you a 1098 for interest you have paid.  When selling real property you will have to provide it to the settlement agent.  

Insurers, Hosiptals and Doctors.  No law requires the use of your social security number as your ID number, except for Medicare, Medicaid, or other government sponsored care).  Some property insurances companies now require a credit check as a part of the application process.  You can opt to provide your social to them or find a company that does not require it.  Private medical insurance now uses identification numbers which are not your social security numbers.  Maybe one day our government will catch up and reduce the risk of identity theft and change the requirement for Medicare and Medicaid.

Child Support.  The Family Support Act of 1988 requires that the parents social security number be collected to issue a birth certificate.  Yet the law allows this to be waived for "good cause" and wouldn't you know it, "good cause" is not defined.

Universities and Colleges. While likely unnecessary to provide, those that accept federal funds are covered by the Family Education and Privacy Act of 1974 which prohibits them from releasing information on their students including social security numbers (they can though release basic "directory" information, name address and phone numbers).

Some tips, ask for and read the group or businesses privacy policy before you give out your number.   Ask why they need your number, often it is just a piece of information they would like, but don't need (except in lending).  If they absolutely insist on a number, and you do not want to give your real number (and you are not talking with a government agency, or trying to commit fraud), you may want to consider giving a fake number.  078-05-1120 was printed on sample cards inserted in to hundreds of thousands of wallets during the 40's and 50's.   It is a widely know as fake, but often not by clerks who are asking.