I'd say it's pretty much a crap shoot, since they are on system status. Some of the northwestern units are based out of a fire station (Medic 316, 317 and others, I'm sure). With most of the county, units move all over the place. Here in Corona, we get units from Moreno Valley or Elsinore quite a bit. 357 seems to be one of our regulars, particularly at night.
The only reliable information is where the unit originated from. The 300 series are from NW Riverside County. I want to say that the 400's are from the Temecula area, but I'm not positive......