I did 10 for my HSC - Adv English, Ext 1 math, Ext 2 maths, Chem and Phys.
I chose very early on to just keep 10 units and not keep Bio as well and have 12 units. Main reasons for me: if I kept both Ext2 maths and bio, I would have offline periods which means I have to wake up early or stay back late or miss out on lunches... this would have also led to reduced sleep (which in yr 12 you need the most of) or missing out on the start of after school tutoring. Also, bio isnt as high scaling as my other subjects and I wasnt as good at it anyway...
Another reason for me having 10 units is that it means I can dedicate more time to fewer subjects which means I could get more study done and worry less about having to study for an extra test when test blocks come round.
In summary, it really depends on your situation and whether you feel that one of your 10 units might let you down, then it would probably be best to go for 12 units, otherwise, if you are happy with your performance in your 10 units, then keeping just 10 units might give you an advantage.
Sorry for the long message,
cya