Join If: You're less concerned with privacy than you are with saving money. These programs are free to join and entitle cardholders to members-only savings on selected products. In exchange, the stores keep tabs on what customers are buying. However, the stores aren't interested in any one person's shopping habits, says Michael Sansolo, senior vice president of the Food Marketing Institute, a supermarket-industry trade group. Instead, they study different segments of the shopping population. For example, stores may want to know about those who shop several times a week, those who regularly spend $100, or those who buy wine, Sansolo says. Keep in mind that data from grocery-club-card purchases can be subpoenaed in lawsuits (including divorce proceedings) and requested by law-enforcement officials investigating a crime, according to Beth Givens, director of the nonprofit Privacy Rights Clearinghouse.

Options

Of the nation's four biggest grocery chains — Wal-Mart, Kroger, Albertsons, and Safeway — only Wal-Mart doesn't offer one of these programs.