Whether you should wash prewashed salad greens is debatable. Some experts, including the Food and Drug Administration (FDA), state that there's no need to wash them. The greens are processed in facilities that are typically more sanitary than the average home kitchen, and washing at home (or in a restaurant) can increase the risk of cross contamination from other foods or surfaces.
Nevertheless, other experts do recommend washing prewashed greens as an extra measure to further reduce any microorganisms that may remain. It can’t hurt, they say, as long as your kitchen and hands are clean—and it may help.
Whether or not you rewash bagged prewashed greens (sometimes labeled "triple-washed" or "ready-to-eat"), keep them refrigerated and eat them before the "best if used by" date.
Since prewashed greens are expensive, you may be better off buying unwashed greens if you plan to wash them anyway.