Auto makers have made great strides in corrosion prevention technology. BUT road salt and even worse, brine, WILL shred your car if you aren't diligent in keeping salt off of it. On a brand new car with a factory perfect underbody paint, less washing may be survivable. Keep in mind however that the rust that actually causes vehicle damage might not even be on the body. Fuel lines, brake lines, ball joints, u-joints, springs, steel gas tanks, gas tank straps, exhausts, steering components, etc (list goes on forever) are often steel and susceptible to rust. Even aluminum parts aren't immune to salty water.
Long story short, if you're concerned about your vehicle rusting, you can't wash it too often. If you do it at least weekly it'll significantly slow but not stop the losing battle with corrosion.