GOOD TIMING: Some Wall Street wizards say the secret to investment success is knowing when to sell. . . . Buck A. Mickel, a director of Irvine-based Fluor Corp., picked a good time. . . . He sold 56,835 shares of Fluor for $45.375 a share on July 24, pocketing $2,578,888. . . . Nine days later the company’s stock plummeted, in part in reaction to the Middle East crisis. If he had waited until yesterday to sell, he’d be about $500,000 poorer.