In my honest opinion - yes. And also no.
Real military vehicles aren't allowed to just rust away as, and contrary to what you'd be led to expect by the efforts of WW2 modellers, vehicle maintenance and repainting is quite a big thing in armed forces. That said, it is 40K where everything is grimy...
However, the rust isn't in realistic places it just appears to be spread over the models in a random fashion. Vehicle rust appears where water is trapped or runs, or where the paint and primer barrier has been breached by damage. Hence rusty road cars have rust speckles around wheel arches, bottom of sills where drain holes get blocked and around panel damage and dings.
Also, assuming we are talking about a similar situation to the modern day where exhaust pipes are usually quite poor quality metal, those exhausts should rust before any body corrosion occurs.
The rust is done very well though, what's the recipe for it?
Coop