http://enterprisecraftsmanship.com/2015/06/08/make-hard-coding-your-default-choice/
To my experiences, I divide this kind of "magic number" into three categories:
#1 Very small chance to change: Hard coding
#2 Good chance to change in the future:
Hard coding with default value but checking config file first
string ConfigSetting_SMTP_host = ConfigurationManager.AppSettings["SMTP_host"] ?? "google.com";
#3 For sure change (such as dev vs product environment): read from config file directly
