Chemistry definitely. I can't stress this enough (which is why I'm repeating what others have said). I didn't understand, tbh, most of prelim chem, and so now am struggling more than what I would like and am finding that I'm having to re-teach myself the things that we covered.
Bio...not so much. Honestly I can't remember anything probably of prelim, however it is forming a base for yr12 work, which is the same for all sciences really (and probs most other subjects too).
Physics, it's drumming in how to manipulate and use formulas (remember them!!), as well as build a strong base with simpler ideas for more complex ones that will require this previous knowledge and understanding.
Basically do your best anyway, not just because knowing the content helps, but you're also creating learning habits that will continue on in yr12. Get them good now, and it'll save you a lot of work, marks, stress and tears later (small personal experience and seeing it in a lot of friends/classmates)