I think preliminary Biology is pretty useless in HSC Biology as a whole, although there are somewhat minor areas in the preliminary course that are recovered in more depth in the HSC course (like meiosis). That being said, my school has only completed two HSC topics so I can't completely confirm that prelim is rather 'useless'.
It's the same with chemistry I guess. You don't really need the content in the preliminary course except knowledge of the key terms, such as moles, molarity, molecules, compounds etc. and formulas. I don't think I remember a lot of content learnt in the preliminary course otherwise. Again, I've only covered two of four HSC topics.
For Ancient History, you don't need to know anything in the preliminary course. People have told me the same for Studies of Religion I & II.
The only subjects I have felt that requires good knowledge from the preliminary course is Mathematics. MX1 students have more to retain in their memory.
So.... if there's a subject you want to slack off in, it's NOT math. :\