Washing lettuce

[Spiff]

In Cecil's answer to whether dead vegetables excrete, I was surpised not to see him comment on the questioner's mother's assertion that iceberg lettuce doesn't need to be washed because it is the one variety that does not contain sand.

Cece, you coulda educated the masses by reminding everyone to wash their veggies, even the organically grown ones, because they all have the potential to be awash in dirt, which most certainly has animal fecal mattter in it. And that stuff can give your GI tract some really nasty blue funk.

http://www.straightdope.com/columns/020419.html

But the real reason I'm mentioning this is that I was not aware of this supposed characteristic of iceberg lettuce. Is this a widespread belief, or is Steven's mother the only person (or one of the few) to have this belief?

Spiff