You should only keep your Yr 10 notes if your doing Australia as your national study - honestly, no one does Australia.
In regards to year 11 notes, depends what topic you do in the preliminary course and the HSC course.
For example, In yr 11, I did Decolonisation in Indochina up to 1959 and in yr 12 I did Conflict in Indochina 1960 to 1975.
It saved us from doing the background to the course in yr 12. So I recommend you keep notes that will be relevant to you in the HSC course.