..."Since my layout represents the PNW in the post War years ... cars common in Florida are much less likely to show up than cars common in the PNW. This works - for me.
We haven't talked about this recently, but your generalization isn't really correct for a number of car types.
First, of course we're talking about "foreign cars" since home road cars can be anywhere from 25% or 75% or more depending on the road and location. But even there, it might be car type specific. PRR averaged 50% home road, but if you split it by car type it might be more like 25% boxcars, 50% gondolas, and 75% hoppers.
With respect to foreign hoppers, the nearest neighbor rules seem to apply. With respect to gondolas, that traffic is often regional, so again, your rule of thumb applies.
Your generalization breaks down with respect to boxcars. The two largest boxcar fleets were PRR and NYC. I would expect to see NYC and PRR cars as the most common foreign car on ANY layout representing a continental US main line.
There are obvious exceptions such as branches that served limited industries and assigned pool-service cars. And no, the car service rules do not seem to impact this very much.
Reefers would also be a car type that might not really follow your rule of thumb. For example as Bill Welch often pointed out, "our companies" reefers (WFEX, BREX, FGEX) were used as a pool of cars and went were needed. I would expect, if you were modeling any sort of major PNW harvest season, that FGEX cars would be the most common of "our campnies" reefers.