Consumers who have asked me whether they should give their zip code at the register have been getting bad advice! I was under the misimpression that zip-level data was only being collected for demographic research purposes (to determine where stores should be located, and advertising directed, on a mass scale) and thus said that no harm came from revealing the zip. No longer. Here's a summary of data practices at William Sonoma, according to a recent California case (Pineda v. Williams-Sonoma Stores Inc., Cal. Ct. App., 4th Dist., No. D054355). Giving the zip code allows the store to "enhance" the information they already have about your (your name from the credit card) and determine your home address:

Jessica Pineda visited a store in California owned by Williams-Sonoma Stores, Inc. (the Store) and selected an item to purchase. She then went to the cashier to pay for the item with her credit card. The cashier asked for her zip code, but did not tell her the consequences if she declined to provide the information. Believing that she was required to provide her zip code to complete the transaction, Pineda provided the information. The cashier recorded it into the electronic cash register and then completed the transaction. At the end of the transaction, the Store had Pineda's credit card number, name and zip code recorded in its databases. After acquiring this information, the Store used customized computer software to perform reverse searches from databases that contain millions of names, e-mail addresses, residential telephone numbers and residential addresses, and are indexed in a manner that resembles a reverse telephone book. The Store's software then matched Pineda's now-known name, zip code or other personal information with her previously unknown address, thereby giving the Store access to her name and address.

So, when they ask for your zip code, say no, or to have fun, give them the zip code of the White House: 20500.