I generally don't buy stuff like this, but I sometimes do.
Whenever I do, I never trust the "pre-washed, ready to eat" label on the package, but always wash it like any other produce.
(No, I'm not saying that all you need to do is to wash this stuff. But I think it is always prudent to wash this stuff if you are going to eat it.)
I read a story some time back where a farmer who was growing vegetables for a company with a similar product was also raising animals on another part of his farm. He was using well-water to wash the veggies, and the animal waste got into and contaminated the well water, which in turn contaminated the packaged "pre-washed" items with e. coli.