I've seen plenty of rust free cars from the northeast.... Lots of people here with nicer cars have winter beaters so the cars that aren't driven in the salt don't rust and they don't have to deal with the southwest sun bake.
Actually, sometimes you never know. I was in a yard five years ago that was being s****ped, it was like a jungle in there, so overgrown some cars you couldn't see in the wintertime from 25 feet away. In one row was a '60 Oldsmobile. For some reason the loader guys there in February picked here and there for cars and they went after this one. It came out in pieces, although partially because it was frozen to the ground. Four or five cars down is a '59 Buick 2-door hardtop. It didn't come out until August or September. In fact, one day a guy showed up who said it used to be his car until 1973. It was a local NY car. Well, when they picked it up with the loader? I wouldn't call it rust free, but all the floors were in it, no signs of frame rot - it was a good buildable car right up until they smashed the roof down so they could put another car on top of it. There were a few others like that, one '55 Ford 4-door looked almost like you could have washed and waxed it and put it on a car lot. Was a real shame some of what got pressed - and yet other cars even in the summer came out in pieces. But the yard also had a high side and a low side, and the low side got flooded at least twice, once the spring of that year. That rotted those cars more than maybe they otherwise would have.
California, Arizona, New Mexico and Nevada. My 57 Dodge was originally from California when I got it (now resides in Arizona) and Over 50 years later when I stripped it down, the only rust that could be found was about the size of a quarter in the bottom of each door and it hadn't made it all the way through yet.
We lived in California for a while in the 70's and I was in heaven after living in Pa all my life and putting up with rusted cars (rusted so bad the frames were half gone). While in Ca I bought a 70 Cad Sedan Deville that was mint and took it back to Pa when we moved back. I would come out of a grocery store and find a bunch of people standing around it amazed that an older car could look that spotless after 12 years. In Ca it was just another used car. Don
Usually rust free cars come from shops and garages. Telltale signs are deliveries of large packages, welding machines and other fabrication tools, and the sound of cutting and grinding. I've seen plenty of rusty California cars. For me, Central California, and Northern Nevada have been good.