Obviously not. It is a waste of time unless you actually want to learn 7 subjects for pleasure. I would probably tell people to do six subjects, neglect 2 of them and focus on 4. Don't do that because you don't know what will be your top four though. I did and it worked for me.
That is some pretty bad advice. Like, you should NEVER neglect any subjects, because you honestly don't know what's going to happen. By the end of the year, I really didn't want to do any more chemistry (particularly when it was my last exam to go), and so I backed off chem study in the last month or so. Chemistry ended up in my primary four, so just imagine how much better I could've done if I had done that study.
Ronnie - music is definitely a massive wildcard, and not one you can assume to get good marks in. Unfortunately, if you do want a decent mark, you HAVE to spend a lot of time on it. If you're particularly worried about music affecting your marks, just don't do it.
However, I DID get an ATAR over 90 (sure, I had some nicer scaling subjects), and my music mark was 25. At the end of the day, if music doesn't end up in your primary four, the difference between a 35 and a 20 is about 1.5 aggregate points - that's not a whole ATAR rank, it's about half of one. Doing music won't detriment you that much if you score lower in it, so I think you should do it and not pick up a fifth subject. Instead, devote that time to practising your final program.