If you're in a study and the researchers tell you not to count iceberg lettuce as a veggie serving for the study, please don't count it. If you want to count it for your own health, that's up to you, but if you don't comply with instructions for the study, you make the study less valuable. The researchers have specific definitions they have to follow if they're trying to make sure that the different groups in the study ate similar things, or that your group got more or less of certain nutrients. It's not like you're going to destroy the study by counting lettuce, but you will make it just slightly less trustworthy.
Once you're out of the study, I would say count it if counting it makes you more likely to eat a salad instead of something higher in calories. Or, you could count it as half a glass of water if you're counting water. That way you get "credit" for it as a healthy thing, but you won't risk being misled about how much nutrition you're getting.