I know it's easy to not think about bacteria in your food. We tend to associate bacteria with uncooked meats, like salmonella with chicken for example. However, after the salmonella outbreak in peanut butter and bagged spinach a few years ago, we have to remember to wash everything. I know the bag of pre-cut salad says "triple washed," but it should be washed again at your house. Thanks to my lovely OCD tendencies, I wash everything already, but I am sure plenty of people just accept that the pre-wash was good enough.
If you don't wash your salad, check out what could be lurking in there. On SoupBelly.com, the author found a dead fly in her bag of salad. Gross!