(I felt the need to fill all my preferences up - if you're wondering!)
It's good to fill all your preferences up so you've got at least something. The risk you take not doing this is if you don't get an offer for anything, you're in a bit of trouble and will have to rely on second+ round (fyi, most offers are done in round 1) so the likelihood of getting something (especially if the course is competitive) is slim.
This is all based on spots available vs. your order/ranking in the uni selection process. Obviously you all know your ATAR now so you have a good idea as to whether you'll get in or not.. but there are only a limited number of places per course (
especially if the course you are applying for is CSP), so it is
super important to have backups and not be cocky and rely on the fact you are destined to get into a certain course.
Another reminder that you'll only be shown the top preference for which you've received an offer for. So you might get offers for all your preferences in the first round (each application is treated individually) but only the
top one will be shown/actually given to you (per round).
I remember when I did my preferences, I filled mine up with courses at unis I had no desire to attend, but the reality is that you can't just rely on previous year clearly-ins and be satisfied that putting 2-3 preferences will be enough. Do all of them, give yourself a big safety net. You can always transfer out after a semester/year (subject to the other uni's entry periods). Unless you have been offered a scholarship or an early offer in writing, nothing is guaranteed.