I did "Human Performance" last. The only reason being as you get to the skills test, there's a good chance that you've not done all of your exams. If it's a toughie left to go, then it just adds more pressure.
I had 4 weeks to run and 3 exams left to do. Luckily one of them was human performance which made it a bit easier. Had it of been 3 tough ones I wouldn't have got my license complete when I did. Also "Flight Planning & Performance" is a shorter subject than others and so leaving that 2nd to last makes the lead out a little bit easier imho.
Although I'm not sure if there's a requirement to have all exams complete before taking the skills test, I have a feeling that there is though.