> 100% coverage doesn't mean much

That's absolutely right. 100% coverage doesn't say much. In fact, by itself it's a pretty meaningless measure of code quality. But the top-down idea still holds. In practice bugs are more likely to happen at the seams between components, and therefore it's where one should test first, not last. It of course depends on the type of the project but I believe it's true for the majority of projects out there.