SJS:
Not to get into that specific point, but it really depends on the Chazaka of the lettuce @ home or on the road.
For example, Dole prewashed iceburg has a Chazaka of being clean and there is no need (as per my Rov) to check each leaf. He does say one should check 3 handfuls (to maintain the chazaka? IDK) and if those have an issue, each leaf must be checked.
And he is known to be a machmir in these issues.