Yale's website is very clear on the issues:
Frequently Asked Questions - Standardized Testing | Yale College Admissions
Q: Some colleges will consider only my highest scores. Why does Yale require a full testing history?
A: An applicant’s testing history provides useful contextual information to the admissions committee. With a full testing history, the committee is able to look at a student's highest officially reported score on each section of the SAT, the highest individual SAT Subject Tests, and/or the highest ACT Plus Writing composite score.
Q: Why do Yale’s SAT and ACT reporting procedures differ?
A: We have taken this approach because there is a difference between the reporting cost of SAT and ACT results. The College Board will send a cumulative record of all SAT Reasoning and SAT Subject tests for one fee. For a similar fee, the ACT will send results for only one test date. Because we do not want students reporting ACTs to incur excessive costs, we require them to send one official ACT score report and permit them to self-report all other ACT results. We will confirm self-reported test results if we feel it to be necessary."
Advice on Putting Together Your Application | Yale College Admissions
"Think of testing as just another part of the application, and certainly do not spend most of your weekends test-taking! Only retake a test if you feel you will significantly improve your scores. If your testing is in the right ballpark, then it probably will not be the deciding factor for your candidacy. In other words, don’t worry about about trying to get that extra twenty points. Instead, spend your time on things that will help you grow as a person: school work, extracurricular opportunities, time with friends — the things that will give you a stronger sense of yourself and, as a result, make you a stronger college applicant. (Bonus: they will also prepare you to make the most of your college experience and of life!)."
