I’ve read (in various books on getting into college) that many AO’s consider it redundant to take an SAT Subject Test for a course in which you took an AP class and did well on the AP test. Your SAT II scores, it is believed, won’t really add any new or interesting information to your application. So that’s something to consider when deciding whether to take SAT IIs and in which subjects.
I guess if you’re a kid who has taken a dozen AP classes and aced all twelve AP tests, and the colleges you are applying to say that SAT STs are optional, you can probably take them at their word.