Good thread! Yup grades and scores matter.
I'd like to add some info about testing.
1. As Low Finish said, most colleges do not Super Score the ACT. They use the composite (1-36) score, plus writing if that is their policy. I am seeing a handful of colleges say they will Super Score the ACT but it really isn't a common practice. it is very common for colleges to Super Score the SAT.
2. Many selective colleges do not allow "Score Choice," that means they want to see ALL scores taken (ACT, SAT all dates, and SAT Subject Tests all dates). That being said, they do admit you based on your best, so they are able to see the lower scores but make admission decisions based on highest.
3. In contrast, many colleges will allow you to exercise "Score Choice" and pick your best test "sittings" to send. In that case you are picking test dates, not best sub-scores. So if you rocked all 3 sections in January but were sick in May and didn't do well, you have the option of just sending January to the schools that allow score choice.
4. To a point, I agree with the OP about taking it early and often. But this isn't the appropriate strategy for every student. Taking the SAT prior to completing Alg 2, and the ACT prior to having some trig, can be self defeating. For slow readers taking the ACT without really practicing the timing on reading sections can be disaster. In soph year, what might be a better approach is practicing on real tests that have been released, and taking them in a timed setting like a real test. (Some high schools sponsor such tests as to prep companies. But be sure it is a released real test, not 'their' version of the test.) Use that practice test to really figure out your areas of weakness, and work on this section(s).
5. For top tier and very selective colleges, you will also want to have between 1-3 subject tests. THOSE can be appropriate to start looking at in spring of soph year. If a student is in honors or AP in a subject that has a test (see the list at the College Board) , by the book published by the college board "The Official Guide to Subject Tests" and have him/her start reviewing now. They should take the subject test in May or June of the year that corresponds with the class. Yup. Prime baseball season, Too bad! (Subject tests are just one hour long, offered most dates that the SAT is offered.) Most kids take subject tests in spring of junior year which is a very busy time.
6. Some colleges accept the ACT in lieu of SAT+Subject tests. This policy is more typical at the smaller elite liberal arts colleges. Handy to have a great ACT score for those schools.
Lots to know on this topic, but the bottom line is to be as prepared as possible, and, to plan ahead. Get the dates and map out your testing plan!