Recommended (not required) by MIT: http://mitadmissions.org/apply/prepare/highschool
However, the typical applicant on these forums, from a high SES background in a well-resourced public or private high school, is generally advised to treat recommendations as requirements (the colleges may be more flexible to first generation, low SES, and other similarly disadvantaged applicants who may not have the opportunities or the advising to complete all of the expected recommendations in time). Frosh applicants of all potential majors are expected to have the well rounded base of typical high school college-prep course work, with more advanced courses and/or additional academic electives in the areas of interest added on top of such a base.