Try your best in all subjects because you never know what the bastards at VCAA will do to you. In life in general I find that if you ever stop doing your best it will come back to bite you on the arse later on and in those circumstances where you were considering giving up but then didn't, you're always glad later on that you didn't.
But, the idea of focusing on your best 5 and then having the 6th bludge subject isn't too bad. I probably never would have done that (then again, I only did 5 3/4s), but if you know for sure that you're screwed on a subject, the little extra increment you'd get from studying probably won't make a huge difference to your ENTER. The question is, then, do you think that you'll need that little difference? Or, will you easily make up the difference in another subject? Will you regret it later on if you don't do your best? It's a difficult situation.
In any case, if you are going to bludge a subject, at least keep up with what's happening and class and do some study, even if its bare minimal. You definitely don't do yourself any favours by 'giving up' entirely. Pay attention in class and do the practice exams and revision material your teacher gives you, at least. You may as well given that you do actually have to attend the class in order to pass it.