that depends on what procedures you're using and for what purposes you harvested the cell. usually log phase or mid log phase is better to harvest lovely growing cells. . the cells on log phase are on their highest volume, so you should harvest log phase cells for harvesting the primary metabolites, such as in transformation or protein expression. Try Environmental microbiology by Maier, Pepper, and Gerba. Several chapters of this book describes growth curve and growth phases. when you want to harvest secondary metabolites you should harvest cells in stationary phase.