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.