G. Mark’s Answer
The obvious answer is that any rigorous preparation will help you with the ACT scores. The question is how to define "rigorous". I would say that anyone motivated to work to prepare for the ACT is already motivated to do the ACT itself. Do you "need" a prep class in addition to those rigorous classes? There are three areas that will get you success in the ACT. First is your studying background. As you mention "rigorous classes", I would say that's a good indicator of your background. The second is the content of those classes and if they cover all the areas that the ACT does itself. The final is practice that will allow you to avoid anxiety or nervousness in a test-taking environment. Prep classes are especially good for that third category, since you'll be very familiar with the situation of taking the ACT.
I've seen statistics that say that a prep class can average a student's increase in the score by 1-4 points. This of course depends on what the situation of that particular student is beforehand. So I'd say that merely taking a prep class will only add to your confidence, and certainly wouldn't hurt. It'll also give you a good indication of what score to expect, and that's got to be good. In my case, I didn't take any prep classes for any of the various state and national tests we took. But that was in prehistoric times when few people at all took prep classes. As for now, the bar has been raised, so I'd recommend it.