DON'T believe everything you read online
Most college's websites purport that SAT Subject Tests are not required, but rather "strongly recommended." And, while it's true that your application will be considered complete and evaluated regardless of whether you submit Subject Tests or not, these subject-specific evaluations are not as optional as many students think.
Colleges' choice to use the word "recommended" instead of "required" leaves room for ambiguity, and with good reason. While admissions officers highly value the chance to compare applicants' prowess in individual subjects, they also know that they come from an incredibly diverse set of backgrounds. Not all students can afford the hefty fees of taking and sending multiple standardized test scores. Not all students have time, between working after school or taking care of siblings, to do their schoolwork and study for additional tests. Not all students have sufficient guidance to even be aware the SAT Subject Tests exist. Admissions offices want to make room for all types of students, including those whose lives may not allow for additional standardized testing. However, if personal and financial barriers are not an issue for you, then Subject Tests are not really as optional as they may initially appear.
DO read up on each school's requirements (or "strong suggestions") for Subjects
Many schools have specific requirements for individual majors: Business applicants should take one Math test, Engineering applicants should take one Math and one Science, etc. Be sure to check each college's website for their Standardized Testing Policies to make sure you can meet all of the requirements!
DO strategize when to take which Subjects based on your AP classes
Most Advanced Placement examinations (which are administered by the College Board, the same company that is responsible for the SAT Subject Tests) include a multiple choice section. There is a lot of meaningful overlap between the AP multiple choice and the Subject Test multiple choice. The question format and overall strategy is slightly different between the two, but studying the content for one will prepare you well for the other.
Many students who are eager to get started on Subject Tests look to take the Biology exam at the end of their freshman year biology course. To these students I say: be wary and take one of the CollegeBoard's online practice tests before signing up. Non-AP courses don't have a nationally standardized curriculum, and there's no guarantee that your school's coursework will cover all the topics that appear on the Subject Test.
DON'T take more than 5 Subject Tests
When asked how many tests each student should take, Grace, one of our College Advisors and former Stanford Admissions Officer warns not to overdo it. "Admissions officers know these exams cost money and take up a lot of time, so there's no need to take ten SAT II's just to show mastery of content. I'd say most students are fine with just two to three, max five. Remaining areas of their application (extracurriculars, summer activities, research work, etc.) are other ways they can show 'mastery' of experience and content that often tell a more interesting story than a one-day exam does."
DO take a good look at the Subject Tests offered on each test date
They're not always the same -- particularly when it comes to foreign language tests. Some dates specifically offer the test with listening, while others only offer it without. But don't worry -- colleges don't prefer one test over the other! Just make sure you're studying and preparing for the right one.
DO take a look at both Math Subject Tests
The College Board offers two Math Subject Tests: Math I and Math II. Math I is for students who have taken two years of Algebra and one year of Geometry. Math II covers the same topics as Math I, "with the addition of trigonometry and elementary functions (precalculus)."
I often have students who claim that their Math skills aren't strong enough to merit even glancing at the Math II. Instead, they only focus on the Math I test. However, if you have completed a pre-calculus course, I strongly suggest looking at both exams, and sitting for a practice test in each, before making up your mind. When I was in high school, I took both tests, back to back, on the same day -- but because Math II is generally curved more generously than Math I, I ended up scoring 60 points higher on Math II. Don't count yourself out for one before trying both!