Did 12 units for my HSC (Year 12).
(Adv Eng, MX1, MX2, Business Studies, Economics and Physics)
By the end of Year 11, I was contemplating on whether to drop Physics or Business Studies so I could do 10 units.
I think at that time, Physics was clearly my weakest subject and Business was like my 2nd strongest or strongest subject (but I was like - its my lowest scaling subject) but I'd think most would say I should have dropped Physics.
Anyways, I went for the safer option of 12 units (possibly dropping a subject by Year 12 half-yearlies).
Fast forward to Year 12, my Physics' rank started to improve (within the band 6 range) and I became more consistent. I had a belief that I could get a band 6 for Physics and I had a thought where I'd get like 91 or 92 for business (since 1st in business at my school the year before got 91 or 92 for business). This means Physics would potentially out-scale it if I got like 90 for Physics and 91 for Business.
In the end, I got 96 for Business (95 exam mark, 96 assessment mark - was 2nd internally) and 86 for Physics (82 exam mark but 90 assessment mark - i.e. I had an internal rank that was in the band 6 range but screwed up the HSC exam for Physics). No amount of scaling would help Physics out-scale Business at that point
(and I got 93 in Economics) so Physics ended up NOT COUNTING.
To be honest, I don't regret doing 12 units either way - Physics was interesting and I don't think I'd spend much extra time on my subjects if I was on 10 units cause I really didn't do too much for Physics.
I was a ''12 units'' person but like people have said, if you're 100% confident in your 10 units - you might be better off doing only 10 (the extra time might prove useful). To win big, you've got to play big and go for it. (But make sure you don't bomb out in a subject
)