Within the context of the HSC course, I think you're only expected to know how to do a line of best fit, except perhaps for a titration curve?
You should be able to tell theoretically whether some relationship is linear or not. For example, you should know that plotting a calibration curve for AAS shows a linear relationship between absoprtion (A) and concentration (c) (the theoretical equation is given by A = εcl where ε and l are kept as constants, I think it's called the Beer-Lambert law). On the other hand, you should know the shape of a titration curve and why the curve is of that nature. So if you were asked to plot pH changes over time, then you would expect to use a curve of best fit (though it's highly unlikely you will be asked to do that).