Setting collection starting dates - how do you decide?

For general stamp collections my starting date is clear: #1 of their national catalogs. The question becomes more complicated when dealing with cinderellas and Postal History. For Catalan cinderellas I look for the oldest recorded ones: around 1890. For Barcelona Postal History the pre-stamp postmark is the first official one from 1716. There are earlier ones, but they're in the museums, where they belong. So far, my oldest Barcelona pm is from 1765. For my Second Estonian Republic Postal History (dormant since many years ago!) I try to go beyon the first obvious date of January 1991, looking for predecessors, with Estonian Patriotic cinderellas on late 1980s. Being lucky to find a few, so far... 

I always like to start a country collection with the mounting of stampless covers (non-adhesives) demonstrating a nice array of cancellations and markings from the first half of the 19th Century and earlier, if available and affordable. Generally speaking you can find nice condition 1800-1850 stampless covers originating from larger cities and towns in the $10 - $30 range, many with content, and some of which can be exploded if the content is legible in the mother tongue and if the penmanship is nice, as it often was back then. Smaller towns and and more interesting transit markings and more points of travel can increase the value, but make for beautiful items to start a collection. 

The reason I like to start with pre-adhesives is that a country collection tells a story, and if the the focus is stamps, I think it's logical and interesting to show how things were done before the first stamps were issued. It's like the forward to any good book, or the appetizer to any good meal.

Cheers!