I actually think its better for people to take the ACT as early as possible. Most college prep course paths will have you taking most of the courses you need to do well on the ACT by 10th grade. I can't remember exactly when I took the test, but I think it was sometime around the spring semester of 10th grade. I know I was WAY earlier than all my classmates.
The worst part for me was fatigue - it's a long test at that age. I'm pretty good at science, but I always felt my scores suffered because it was the last section of the test and I was beat. Of course, the last standardized test I took was 8 hours, so the ACT is a piece of cake compared to that.
I took the ACT a bunch in high school - probably at least 5 times. I was very consistant, every score was the same until the last one, where I scored three points higher. I attribute that to being more relaxed - since I was already accepted to college there was no real pressure. The only reason I took the test was because I had registered for it and paid already.