Updated: Sep 9, 2021
The answer to that question is dependent on the student taking the test and the college that the student is applying to.
If the student is applying to highly selective colleges (colleges where the acceptance rate is less than 30%), then the student should take the SAT or ACT test and aim to get a score in the 95th percentile or higher for that test even if the college is test optional. The Common Data Sets for Harvard and Yale, for example, show that more than 70% of the accepted freshmen for the year of 2020-2021 submitted test scores. That means the number of accepted students who did not submit test scores is less than 30%.
Also, test scores are considered, if submitted, for many test optional colleges. What this means is that if the student has a test score that is above the 75th percentile of a college's prior year test score for accepted students, then it may be a good idea to submit the test score to be a more competitive applicant at that college. A strong test score only strengthens the student's college application. So it would be a good idea for students to study for the SAT/ACT and take the test at least to find out whether the student can get a good score that is competitive enough to use for the colleges that the student wants to apply to.
If the student is not good at test-taking and gets a test score that is less than the 25th percentile test score of the college, then the student should probably not submit that test score to the college. In such a case, the student may be better off applying to test-blind colleges that do not consider test scores from any applicant. Some good examples of test blind colleges are University of California and SUNY schools in NY. However, I must mention that despite the fact that test scores are not accepted by test blind colleges, admissions officers may still have access to the AP national exam scores for a student that can give admissions officers a good idea about the student's test-taking abilities, even if SAT/ACT test scores are not provided to the college.
To answer the question, students should attempt the SAT and/or ACT at least once in their junior year to see if the student can get a good test score. If the test score falls above the 75th percentile of the college's prior year test score range, then the student can submit the SAT/ACT score to the college. A good test score can increase the chances of the student getting admitted and a bad test score need not be submitted for test optional colleges.
Independent college admission counselors can help students determine the best list of colleges where the student's test score will be considered competitive or above the 75th percentile and thus help the student to apply to colleges that are a better fit so that their chances of getting in are higher. In the same way, counselors can help students with lower test scores by providing SAT tutoring or by preparing a list of colleges that are a good fit, where the test scores can be omitted and the student still has a chance of getting admitted without test scores.