U.S. companies recalled bagged salad greens over 8 times in 2012 due to contamination with Listeria bacteria. Listeria can multiply at refrigerator temperatures, and cause deadly blood poisoning or meningitis in older adults and people with weak immune systems.
Should you wash bagged greens that say "washed" on the label? No. Washing bagged greens that were already prewashed does no benefit, according to FDA produce-safety expert Michelle Smith. "Once disease-causing bacteria become attached to leafy greens, it's difficult to remove them by rinsing with water."
She goes on to say "The greater likelihood is that you'll make a safe product unsafe because of cross-contamination with bacteria from your fingers, cutting boards, countertops, or the sink."
Listen to Michelle Smith. Do not wash salad that has already been prewashed. However, with unwashed salad, you should do as you normally would and wash them.