Show New Yorkers a checkout line and they’ll tell you whether it’s worth the wait. Starbucks at 9 a.m.? Eight minutes, head to the next one down the street. Duane Reade at 6 p.m.? Twelve minutes, come back in the morning. But now a relative newcomer to Manhattan is trying to teach the locals a new rule of living: the longer the line, the shorter the wait. Come again? For its first stores here, Whole Foods, the gourmet supermarket, directs customers to form serpentine single lines that feed into a passel of cash registers. Banks have used a similar system for decades. But supermarkets, fearing a long line will scare off shoppers, have generally favored the one-line-per-register system