My eldest child will likely apply to some of the very selective universities that require SAT Subject tests. He is in 9th grade. I plan to have him take subject tests each year in June for classes he will not have again until 12th grade (or ever). His school follows the typical biology/chemistry/physics sequence for grades 9-11. In 11th grade he can take AP Biology or AP Chemistry in addition to physics. In math his sequence is geometry, pre-calculus, and calculus for grades 9-11. He is taking French each year. For social studies the sequence is World History I and II in grades 9 and 10 and U.S. History in grade 11. So an SAT subject schedule could be as follows:

grade 09: Biology Molecular and Mathematics Level 2 (he is advanced in math and is taking pre-calculus outside school)

grade 10: Chemistry and World History, maybe French or SAT Math Level 2 again

grade 11: French, Physics and U.S. History. Maybe English instead of French if he does very well on French in grade 10. Maybe Biology and Chemistry again if he is taking AP classes in those subjects and the previous scores were not very good. Maybe Math Level 2 again

grade 12: maybe tests in classes being taken by the November test date (so they be considered by admissions), if there are not yet enough very good scores

My understanding of Score Choice is that he will be able to choose which subject test scores are reported to colleges.

Here are the subject test requirements of a few schools:

SAT Subject Test in Mathematics Level 2 (official scores from testing agency)
1 SAT science subject test: biology (ecological), biology (molecular), chemistry, or physics (official scores from testing agency)

Same as Caltech, except that Math Level 1 or 2 can be submitted.

Two or or more subject tests. A score of 700+ on a language test meets the foreign language requirement.