You can get a good grade — even a 5 (the top AP score) — without having to correctly answer all the multiple-choice questions presented to you. In fact, if you get only half the multiple-choice points and score big on the essays, you can walk away with the top grade. That said, you really need to stock up on knowledge about social trends and movements, which is particularly useful for tying together essay responses. The good news is that this information also helps you answer multiple-choice questions.
The AP U.S. History exam is center-loaded, with a bias toward post-colonial history between 1800 and 1950. That doesn't mean you should neglect the way-back and only-just-recently events; you have plenty of points to gain from these time periods as well. But as you review, put a special polish on the time periods with the most credit attached.
Historians have a natural aversion to recent events; when happenings are still news, many don't consider them history yet. Sorting out the importance of recent events is difficult. For instance, Lady Gaga gets a lot of media coverage, but does that mean she belongs in the history books? Also, the AP test occurs in early May, and school often runs into June. The College Board is kind enough to not test you on material you haven't covered yet. So don't spend much time on last year's hot topics; the AP test probably doesn't know they exist yet.
Why do the 300 years of early American history get less coverage than the 200 plus years after the birth of the United States as a nation? Well, this is U.S. History, and the U.S. didn't exist before 1776. More important, history is what humans choose to save to tell the later generations about themselves. Historians believe people can learn more about their present selves from studying, for example, the rise of democratic ideas in the 1800s than from analyzing cod-fishing stories from the 1600s. But you don't have to give up on that 20 percent of credit from before the Constitution; it's not just fishing stories.