Yeah, last time I figured out a fair but sensible way of doing this. On route 1, for example, Rattata and Pidgey will be present and common (i.e. easy to catch). There will also be one or two pokemon from similar routes in other regions, such as Sentret, Zigzagoon or Bidoof. These pokemon will be classed as 'rare' however, meaning that the standard of post needed to catch them is higher.