Is it worth tearing lettuce for salad?
Tearing is NOT worth the extra effort, tested experimentally.

Others have explored the theoretical reasons behind this, so I decided to test it in real life.  I did this like so:


Green leaf lettuce from the local CSA
Cut one leaf with a sharp knife (stainless), and tore the second leaf carefully by hand (fast, clean tears)
Pieces were both wrapped in moist paper towels and left out at room temperature
Photographs were taken at 30 minute intervals


After 2 hours, the 2 leaves still look identical (picture below).

Cut (on the left), Torn (on the right):



Provisos: knife was sharp and leaves were fresh and kept moist.  A dull knife or more abusive handling of the leaves may yield bruising and faster browning.  I am currently repeating this on a longer time scale, and may attempt with a dressing.



Practical take-away from the experiment:

I have found that in real life, it doesn't matter how you divide your leaves, as long as you don't apply dressing before storing for a prolonged period.  The acidic dressing will wilt leaves and cause discoloration, and leading to a limp, soggy salad.  The experiment supports that neither method leads to unacceptable browning within a short timeframe.  In practice, I've only seen browning when lettuce is left overnight or longer, or when it is stored with dressing.  

Cook's Illustrated confirms this with their experiment to see if plastic lettuce knives are worth it. They found that: 


  Though all lettuce began showing some browning on the ribs after 10 days, none showed any signs of browning on the cut or torn surfaces. After 12 days, the heads cut with metal knives showed faint signs of browning on these surfaces, and the lettuce cut with the plastic knife followed a day later. The torn lettuce was last to brown on its ruptured edges, starting to turn at 2 weeks.


Given that lettuce is browning on the ribs before it does on cut edges, the difference between cut and torn is no longer important.  You're only adding 20% to the time before browning appears on cut surfaces.  Most people will agree that after 2 weeks lettuce no longer has its full flavor. I have found that to get the freshest, best taste it should be eaten no more than 4 days after harvest. 

So, if there difference between cut and torn is insignificant, why cut rather than tear?
It's faster and more consistent, especially if you have good knife skills and a sharp knife. You can cut a head of romaine in under a minute by knife, versus several times this by hand.  You also don't have to worry about bruising leaves or making irregularly shaped pieces by accident, and you can choose to do large pieces or fine, fluffy shredded mixture.  In my time in professional kitchens, I came to rely on the knife as an extension of my body, and you should too.