Criteria For Obtaining Certs

What is your criteria for obtaining certs? Is there a particular catalog value, issue date, etc that will trigger this action. For me, collecting stamps prior to 1920, I would be obtaining a cert on any unused US stammp prior to the 1903 regular issue, with or without gum and with a catalog value of $250 or greater. The same would apply to used 19th century US stamps. For foreign, I would use a cut off date of 1910 and a minimum catalog value of $100. What is your criteria?