If you think you can handle the work load, then keep all of them. That way you have more "back up units" if you don't do as well in the HSC as you might have liked to, therefore allowing "breathing room" for your marks. Think about it; you drop down to 12 or 11 units, excluding English, you will only have 1-2 units back up in case you do really terrible in one subject. If you get me.