The only thing im scared of is my accuracy going down. I guess it will improve over time.
Don't be. A bit after the system was put in last year, theymos revealed in the donator board that an accuracy of roughly 60% was above average and that anyone reporting more than just the extremely obvious stuff would likely have an accuracy below 90%. People who have been local moderators for a board for some time (so they still have to report things outside of their jurisdiction) have an accuracy >95%, but <98%. Obviously, it's a private board, so I can't tell you the exact figures, but that's roughly what the figures were.
The raw number of good reports is much more important than the accuracy, as long as the accuracy is reasonably high.
Is reporting necessary for posts in the wrong section?
Yes, we would greatly appreciate it.
If it is, should I provide the correct subforum in the textbox
Yes! You don't have to, but it really makes it easier for us since we only have to look at the post to verify that you were correct instead of figuring it out by reading it more closely. It saves several seconds per report. Not only that, but it shows your sense of judgement for where things belong. A person who states where a thread should go and is usually right about that is much more likely to be made a mod than someone who always just puts in "wrong forum".