Some of the more selective universities like Harvard, Yale, and Princeton now require SAT subject tests. Check with the admissions departments of each school to determine which, if any, test is required. These required tests are essential for you to take if you are planning to apply to that school. Math-heavy schools like California Institute of Technology will often require at least one math test. Liberal arts schools may require the writing test.
When you have discretion to choose your test, gauge your strengths carefully. Be sure to differentiate between your personal interests, and subject tests on which you are likely to score well. Your favorite subject may be world history, but if you have not taken a world history course since 10th grade and you are currently receiving an A in chemistry, you might be better off taking the SAT Chemistry test.
High school students are frequently stretched thin. Between regular school assignments, sports, other after-school activities, and college visits, standardized test preparation time can be scarce. Try to avoid choosing a test that will require extensive preparation time that you simply might not find available as the test approaches. As with any standardized test, repeated practice of the test leads to better scores.
Without fixed requirements or substantial performance differences, you may consider subject variety as a final determining factor. An admissions department may be impressed with a student who scores well on subjects as diverse as physics, literature, and Latin. When students are doing as much as possible to stand out from the crowd, demonstration of a wide array of academic talents can only help your cause.