Etiquette for anything to do with seat recline, arm rest, lockers etc all comes down to the people you are either lucky or unlucky to be seated with.
Yes we can all well say it's the "right" thing to do when flying with whatever the majority think...but it's going to do you no good in the air if someone hogs your space. What do you say? "I read on a FF forum that you should not be using the arm rests as I'm in the middle seat". It's not going to help in any case.
Some people have manners and decency, some don't.
I don't see why these threads end up with pages and pages of discussions time after time, when sometimes it just plain sucks to be seated next to someone who has no respect for other travellers. And this applies for any seat you pick, even sometimes in Business (not the middle seat issue, but plenty of similar ones)