How long are you planning on keeping the car?
I thought BMW gave us a 12 year / unlimited mile rust and corrosion warranty. If you keep the car 12 years, it's gonna need a lot more than a de-rusting.
Also, some of the rust-proofing crap they spray under there tends to degrade rubber.. so if they spray your bushings a bit too much... eh. I'm not worried about rust. I live in NJ, and just give the car a semi regular bath in the winter. My coilovers aren't rust proof (and neither are most springs and shocks) and they are very exposed, so I would just check on those every once in a while to see if there's any surface rust.