First Day Indicias -

I am in a quandary relating to FDC indicia. Most covers that I own have the 'First Day of Issue' in the killer bar cancellation. I (think I) know that this was not the case for very early issues. I have recently been looking through boxes of First Days and some have the cancel date for the first day but without the killer bar wording. Also, I have come across several First Day Postmarks from Post Offices which were not the 'Official Post Office' of issue. From a collectors point, are these a 'category' for the First Day Collector? I collect FDC to complement mint and was wondering if I need to set some different parameters for that part of the collection. Is there any history on early FDC's for the beginning of killer bars w/First Day of Issue noted