Seriously: never handle robbery as a simple transaction -- many times the robbers do realise in their aftermath that they should in fact have killed you, and you don't want to be still around then. (I've tried to link a good post here, but it's pretty hard to Google this subject. I'm not an expert, so handle my comment with caution.)
Well I agree that you should get to as safe a location as possible immediately following a robbery, however I really would like to see the post you had in mind. Maybe I have a different percentage in mind when I read the word "many", but I'm highly skeptical that this is a common occurrence amongst robbers. Granted they tend not to be the best critical thinkers, but why would they risk adding homicide to the list of charges should they be caught?
If the late killing really often happens, which I (not based on any data or anecdata) doubt, I would think the more likely mechanism would be that they decide up front, hesitate at the moment of truth, then realize you could recognize them later, making them go to jail for a long time, and then do it anyways.
The longer an encounter with a criminal, the more likely they are to commit a worse crime: both "practical" crimes like killing you to keep you quiet and purely malicious crimes like rape and torture.
Mind you, we don't know the heart rate of the robber.