Should I wash my pre-washed greens?
I think that washing pre-washed greens is an issue of emotional security, if you don't trust the purveyor.  As the FDA indicates:


  Many pre-cut, bagged, or packaged produce items like lettuce are pre-washed and ready-to-eat.  If so, it will be stated on the packaging.  If the package indicates that the contents are pre-washed and ready-to-eat, you can use the produce without further washing.


It certainly does no harm, but you then have to also dry them for most uses, so it is more work, and kind of redundant:  you have paid a premium price to buy the greens already washed.  If you are going to wash them yourself, why not buy less expensive produce, and perhaps have more control over what goes into your mix?