Everyone is on the right track here.
Essentially, most subjects scale down and only very few subjects scale up.
When a subject is described as a "good scaling subject" it refers to the magnitude and severity of the scaling. That is,
how much does the subject scale down. The smaller the difference between the HSC mark and the scaled mark, the "better" the scaling.
UAC's table A3 is the best source of information:
http://www.uac.edu.au/documents/atar/2011_HSC_Table A3.pdf
A quick look at the "mean" column shows that only Maths Ext 2 "scaled" up for HSC 2011. Latin Ext. use to scale up but last year's results show likewise. Also, distinction courses such as cosmology and philosophy normally scale up as well (not shown in table A3 since they are run by universities, not BOS)
You should also look out for the P99 and P90 columns.
Using Aboriginal studies as an example:
P99: If a student got 48/50 (the top 1 percentile of the cohort/ better than 99% of the cohort ), scaling reduces this to 41.7/50
More disastrously, P90: If a student got 44/50 (top 10 percentile/better than 90% of the cohort) scaling reduces this to 32.1/50
Hence Aboriginal studies is a bad scaling subject.
While table A3 may depress most people once they calculate the total aggregate points that they receive (out of 500), bear in mind that your ATAR is a RANKING.
99.95 means that you are in the top 0.05 percentile of the grade, that is you are better than 99.95% of your HSC cohort.
Anywhere between 470-480/500 is normally considered to be the benchmark for a 99.95 ATAR. This varies year by year depending on how well the cohort does.
Hope this reply isn't too long and clears up any confusion.