I am aware of this question and of the CRC's lenient policy on lettuce without a hechsher. However, everywhere I've been, it is assumed that one needs to check (pre-washed, even triple-washed) bagged lettuce if it doesn't have a hechsher on the bag. I proceed on the assumption that it is the minhag, if not the halacha, to do so.
My question is instead about pre-checked lettuce with a hechsher on the bag, which one can sometimes find, even from mainstream brands and in regular stores. Strictly speaking, does one need wash and/or check this before use? A few points:
My kosher grocery store carried a sign saying something like "Bagged lettuce without a hechsher on the bag must be checked" (perhaps implying, perhaps correctly that bagged lettuce with a hechsher doesn't need to be checked).
However, a friend of mine who knows her way around a kosher kitchen suggested to me that even bagged lettuce with a hechsher should be checked (although she didn't seem particularly sure or emphatic about it).
There is also the consideration of whether lettuce needs to be checked in principle, even if only by cursory inspection.
Can anyone bring sources to answer this question definitively? Does commercially available bagged lettuce with a reliable hechsher need to be personally checked before it can be used?