Bagged lettuce may seem like one of those true conveniences of modern life. You want to eat more salad, but don’t want to spend time washing, tearing and chopping. But is eating bagged lettuce as healthy for you as a good old head of lettuce? This week on “Take Care,” Kerri-Ann Jennings , a registered dietitian and freelance nutrition writer, answers that question for us. Jennings contributes to WebMD, FoodNetwork.com and other publications.