Tabula Rasa? When to Start With a Clean Slate

I specifically liked the point made on page 22 of Meeting Minds: A Brief Rhetoric for Wrtiers and Readers about how there are certain assumptions a writer makes to grasp his or her intended audience: "when your readers identify an assumption or values they share with you, they will be more sympathetic to you reasoning."

One problem though: To what point does one assume? Surely a writer will assume the language is something the audience members will understand. But what about concepts? Ideas? Words? At what point does one say, "everyone should know this?" And when to we start tabula rasa?