UCs now range from recommending them for certain applicants to not recommending them at all. UCB and UCLA recommend a math and science for STEM applicants but does not recommend subject tests for others, see http://admission.universityofcalifornia.edu/freshman/requirements/examination-requirement/SAT-subject-tests/. For NYU, you would need three SAT subject tests to have any reason to submit them – it gives you the choice of submitting any one of the following: (a) SAT, (b) ACT c) three SAT’s, or (d) three AP’s, see https://www.nyu.edu/admissions/undergraduate-admissions/how-to-apply/all-freshmen-applicants/standardized-tests.html. If you submit a high enough SAT, you won’t need the three subject tests as an alternative.
BU, starting about 5 years ago, went from requiring subject tests to recommending them, to not recommending them but will consider them if submitted, to now no longer even stating it considers them if submitted. The exception is that subject tests are required for its special medical and dental combined undergrad and then grad programs that exceptional students can possbily get into.
UVa, starting this application season, has stopped recommending subject tests and now just states they are “optional,” meanig it will give some consideration to them if submitted. Northeastern neither recommends nor considers them.
It would thus appear that it is not necessary for you to take subject tests, and if the choice is between taking the tests or improving your SAT, the latter is preferred because you should not assume good subject tests scores will make up for a weak SAT.