i dont think any area is better to find a lv.5 chocobo than another. the chocobos i found couldnt reach lv.5 til around chapter 5, and i just went to kilika woods (i get more chocobo encounters there but that could just be me)

also, dont you need sylkis greens to level up a chocobo from lv.4 to lv.5? and you cant get those greens til chapter 5. i think =P