As a fellow student sitting the same exams as you, it's probably best to worry about after this month of exams. Maximising your atar now will expand the many degrees you could get into. However, I could be mistaken about the time-window you have for your preferences, but I doubt UAC is that harsh.
Compared to how most school's mark harshly, do hsc markers adopt that same mentality for marking, or is it vastly different? I'm asking this for subjects such as English, History (and maybe religion), which are mostly subjective.