I'm not going to vote becuase, at least for my history and the fact that I'm not currently working for a company, 'it's complicated.'
First company I worked for:
Central station to clock in, sign out units, and then to posts. I wouldn't quite call it SSM since it was mostly IFT so we knew what was scheduled and where the most active spots were by simply knowing where our contracted facilities were (hence, even the unscheduled calls came from constantly the same locations) and not by a computer model based on call history. Additionally, we had a 4-5 24 hour ambulances based out of motels who weren't moved to posting spots for covered. Additionally, ambulances that were at posting spots were allowed to rove around that specific area. So while the posting spot might be 1st Street and Ave. B, if your favorite food joint was at 2nd St. and Ave. C, no one would complain about it.
Second company: Central base, but we were allowed to roam if we wanted to.