How well a board oversees its executives -- their pay, their performance and the risks they’re taking -- may affect a company’s shares. Morningstar Inc. assigns governance letter grades for U.S. corporations by measuring such things as how well boards monitor executive pay. From Oct. 10, 2007, when the Standard & Poor’s 500 Index peaked, to Dec. 5, 2008, shares of companies that earned A’s fell an average of 33 percent, while the index fell 44 percent; those that got F’s tumbled 63 percent.