I might offer a compromise position. There are some classes where – almost by definition – the class’s curriculum and the AP exam material are likely to have a very high overlap. That would be true for sure on math, and also languages and at least some sciences as well. So it may not take a ton of additional studying to do well on those AP exams if a kid has taken a challenging course in that discipline but not one billed as an “AP” class. By contrast, there can be a huge amount of variance between what a teacher chooses to cover in literature and history as compared to the AP exams, and there I’m hesitant to think that additional study is really worth the effort. Of course even there you could compare the curriculum for your kid’s class to what’s covered in an AP test review book and see if they might happen to match up well.