ACT is not popular in west cost so few students here took it. I don't have direct experience with it. My understanding is that ACT always allows score choice which becomes a competitive advantage in the mind of students. SAT offers score choice now is more a response to ACT's market share gain (I was told by a college application insider).
For the case of Stanford, yes, it requires all scores for SAT and ACT. I get this from its website.
"SAT Reasoning Test or ACT
Stanford requires either the SAT Reasoning Test with Writing or the ACT with Writing to be considered for admission. Please note: Applicants are required to report all SAT and ACT test scores. Your official test results must reach Stanford no later than November 1 for Restrictive Early Action, January 1 for Regular Decision."
My suggestion is that you should ignore score choice. There are universities that don't care about how many times the tests were taken. There are also schools trying to find a reason (any reason) to reject a student. If a student can get 2400 in one shot, it will be the best. :-)
