No.
While statistically, all of the terms have been determined to be important, it's not necessary to use all of them. In most cases, scoring a B+ or better on contextual optimization will have a noticeable positive effect on search performance.
It's also possible for some functional text to occasionally slip through the filters. Text like "Add to cart" or "Next page" shouldn't appear in the Semantic Index. There may be some instances where similar functional text- which isn't directly related to actual content -- does slip through (usually as a result of writers embedding marketing text in the normal text of their articles). In such cases, these terms can be ignored -- although using them may still increase the contextual correlation to top performing searches.
Bear in mind that as with all SEO, Google uses hundreds of additional ranking factors besides contextual relevance. Answering specifically what grade you will need for contextual optimization isn't possible because so many additional variables apply differently to each individual web page. In most cases a B+ minimum is a good rule of thumb.