Convenience may be outweighed by safety when it comes to pre-packaged produce.

Although experts disagree on which is better there is some good advice to help avoid contamination and food poisoning:

Bruhn is also a staunch advocate of irradiation, which she says can ensure food safety.
She encourages consumers to take steps to avoid compromising bagged lettuce. Buy only bags kept very cold in the grocery store and pay attention to sell-by dates. Once you’ve got it home, open the bag and dump it directly into a clean bowl.
“Don’t stick your own hands in there,” she said.
She also urges home cooks not to re-wash bagged greens because of the possibility of cross-contamination with other bacteria already in the kitchen.
If you want to use bulk lettuce, make sure to clean it correctly, Bruhn said. First, wash your hands and also the sink with hot soapy water.
Then, break off each lettuce leaf individually, rinse it under cold running water while rubbing gently. Dry in a salad spinner or with paper towels, not with cloth towels, which may transmit bacteria.