Greeks may not technically use lettuce because it's not as readily available in Greece (I gather) but Greeks who live in places where lettuce is readily available DO use lettuceor not, as the mood hits. My family def. uses lettucethe kind is unimportant. My parents prefer iceberg, but my grandmother doesn't. . .my advice, though, would be not to FLOOD the salad with lettuce if you do choose to use it (as I do).
Agree with what Beety & others say about the dressing. My relatives around Sparta simply use excellent quality olive oil--they don't even bother with lemon, for the tomatoes provide acid enough. It's the most delicious thing I've ever tasted, but alas the quality ingredients here won't allow for the omission of lemon juice or red wine vinegar (which is also OK).
Guess what I'm saying is that it really isn't an exact science.