I'm not sure about the mirror thing. On most traditional arans I've seen, they do mirror each other, but I don't know if this is always the case. I would think that if you didn't want to do that for your sweater, you wouldn't need to.
I'm going to swatch for my sweater this weekend. I have two other projects that are finished except they need buttons, so I am going to pick some up tomorrow and get them to FO status.