"The reason a doctor or a dentist asks for your Social Security number is that, should you die while under his or her care, they are required to put your Social Security number on the death certificate," says Foley.

Even so, fulfilling noncredit-related requests -- even medical-related requests -- is purely optional, says L.

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"As with so many procedures in the business world, your Social Security number is something that many companies ask for, so no one really questions it," says James Van Dyke, president of Javelin Strategy & Research, a research firm that tracks financial services topics.

"But giving out your Social Security number is definitely a practice consumers should think twice about." Case in point: A recent Javelin Strategy & Research report -- the 2009 ID Fraud Survey -- found that, among identity theft victims, 38 percent said the perpetrator had obtained their Social Security number and used it in the crime.

"It's that prevalent." 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.

Group number (shown in black): This number is a relic of paper filing systems.

As the SSA says, "This was done back in 1936 because in that era there were no computers and all the records were stored in filing cabinets.

Potential employers also need it, and they may even want a copy of the actual card, says Linda Foley, founder of the San Diego-based Identity Theft Resource Center (ITRC).

You'll also be asked for it at your local Department of Motor Vehicles, car dealerships, pawnshops, drugstores -- even at the airport, should you lose your luggage, she says.

Not exactly, according to the Social Security Administration.