Well i'm just finishing my hsc this year, and i dropped down to 10 units as soon as i could. Personally i found the idea of doing 12 units as a back up to be a negative perspective, and allows you to fall into the mindset of not having to do that well because you have a backup. This could eventually degrade the quality of all your subjects - particularly with the added stress of not just extra assessments/course content, but also not having study periods to work on your other 10 units. While there is the idea of doing it until it becomes too much for you, by this stage having that extra subject could already have had a detrimental impact upon your other subjects.
Overall i am so relieved that i dropped down to 10 units and it was definitely a factor in allowing me to achieve high rankings. Each individual is different and if you'd feel more comfortable with a back up then go for it, but i just think it's more likely to be detrimental than beneficial. Hope that helped