UCs only look at subject tests in some specific majors and schools, so take a look at that before you decide. For example, here’s the recommendation for UCSD:
San Diego
Jacobs School of Engineering and biological or physical sciences majors: Math Level 2 and a science test (Biology E/M, Chemistry, or Physics) closely related to the applicant’s intended major.
You can find the list of all the recommendations at :
Also:
Recommendations for fall 2018 applicants
Remember, these are recommendations, not mandates. Students will not be penalized for failing to take the SAT Subject Tests.
A few UC campuses recommend students take certain SAT Subject Tests for specific colleges or schools, and majors. Even if subject tests are not recommended, they may add merit to a student’s application.