Maybe save 11/12/13 for the end. They're more like the real exam in question style and might be easier to tackle once you've gone through UWorld. The older exams are little more straight-forward questions and less clinical. At the end of the day, they all test the SAME facts. Some are harder than others, but they are all curved and scaled to provide fairly accurate assessments of how you would have performed if you took the actual Step 1 exam.
I actually have found the old exams to be very helpful. Found bootlegged copies of 1-4, which have been paramount for me to identify and go over recurring, fundamental facts and concepts that Step 1 seems to test on almost every exam.
I bought form 11 for my first one. It was pretty tough, but I ended up doing really well on it. I was personally going to make sure to complete forms 11, 12 and 13 in order to see the most recently released questions, so I just figured I would do them in that order. I had also only completed about 35% of UWorld at that point. It seems like the only form may not be good to complete first is 13 because it's apparently pretty difficult. I wanted to save that until last.