That hasn't been my experience, across 30-some rusty beaters. First to go is the wrap-around weld on the door and hatch edges (especially on fords), followed closely by the wheel arches where salt-laden crud and sand gets jammed against the inside of the steel, after sneaking past the fender liner. Certain AMCs and Chryslers from a few years ago had a big problem with the front fenders- there was an actual ledge in there where salt-laden sand would be packed against the inside of the top of the fender, and stay there till you cleaned it out by hand. With due respect to Japanese cars, which I own one of and basically love, I don't see many older ones around here that aren't totally bananna-spotted with rust. Guess they don't salt back home in Japan, so the engineers didn't spec coated steel or whatever. Now that many/most are made here in NA, maybe that has changed.
I'v had some luck, in years I wasn't too lazy, with saturating the door edges and under the hood with cheap spray wax mixed with hot water. Sorta like the shipping wax the manufacturers used to use. Gotta do this in the fall before the weather turns, however, and it is pretty easy to forget in the rush of real life.
But having said all that- I still get the cars bottom-washed whenever there is a thaw, if it lasts long enough for the lines to die down. And now that I have a garage (non-heated, but house leakage probably keeps it barely below freezing at worst), not scraping the glass in the mornings is worth the increased rust of the temp cycling to me. Neither of my current heaps is anywhere near collectible, and I drive the rusty one when the roads are white. I doubt it makes a significant difference- if sun comes out on a snowy day, greenhouse effect gets my car hot enough to melt off all the snow anyway. Very annoying to come out at 1700, and the doors are frozen from refrozen meltoff. (Also been too lazy to silicone the weatherstrip the last few years...)
aem sends...