The Question: Does it make sense to invest in an ACT tutor before the first official ACT test in Feb of junior year? Or, is it better to take the test (with individual prep through an ACT prep book), see how you score and decide on tutoring after that?
Regardless of how you prep (with or without a tutor) do not take a test “to see how you score”
This is among the worst mistake I see students make.
I liken this to jumping out of a plane to see if your parachute works—why not inspect it from the safety of the ground and jump as few times as possible?
In your case, why not take practice tests and then take the real test as few times as possible?
You do not want a mediocre score on your score report if you can avoid it.
Now, to your question specifically, the decision to use a tutor or self-study (or take a class, etc) should be guided by the number of points you need. The more points you need, or the less time you have, or the less self-disciplined you are, the more you should probably use tutors. If, on the other hand, you have ample time, self-discipline, and only need a few points, then self studying should be sufficient.