It's a waste if you gave up a lot during year 12 when you didn't need to do so. High ATARs are useful for scholarships, high study scores are useful for placing out of classes, but if you only need a 75 for your course and you don't think you'll get a scholarship or be placed out of certain classes, I don't think it's worth sacrificing much to get a 90 instead of an 85, for instance.
(On that topic, I worked hard in VCE for two reasons - I thought I had a shot at some scholarships and needed them to study interstate, and I wanted to apply to universities in the US, which require you to submit a class rank.. and without scoring in the high 99s at my high school, my rank wouldn't have been in the top 10%. I did pretty well in high school, and I remember telling someone that I was applying to study science. She seemed shocked. I then told her that I was also considering science/law. Her reaction? "Oh, that's much better, then.")