Check out the March 2010 Consumer Reports on packaged salad greens. Here is a link to that article: http://www.naturalproductsmarketplace.com/news/2010/02/packaged-salad-may-contain-bacteria.aspx
As always, it's better to be safe than sorry. In this case, you may want to thoroughly rinse your bagged salad greens even when the label says “pre-washed.”
Another thought on salads: do a liitle research before you head out to your favorite restaurant and order one of their entree salads, thinking you're saving calories over the meals you usually order. Most chains state the calories on their Web site. Just because it's a salad does not necessarily mean it's low in fat or calories. For example: a typical restaurant salad may have 4 or more tablespoons of dressing, adding 250-400 extra calories, so always ask for the dressing on the side. That way you can control the amount. Many restaurant dinner entree salads have more than 1,000 calories and that's sometimes before they add the dressing! That's as many calories as a good-size burger and fries. Not that I am rcommending that instead, as the saald typically has a good variety of nutrients and fiber. Just be choosy about what you order and be specific and assertive with the waiter. You can usually ask for additions or subtractions to the salad. For example, I usually ask for the dressing on the side even if it's a light version. I also ask them to leave off the croutons. If the salad has Buffalo or fried chicken, I'll ask if they can substitute with grilled chicken. If the salad is really large, I'll ask for a to-go container and put half in it before adding any dressing. This way, it will stay crispy for my lunch the next day.
Doing a little research is part of the process of pre-planning and being prepared. These are strategies that will help you be successful in managing your weight and your health.