These safety inspections are more for frame and suspension ares, not rusted fenders. If you have a rusted out hole right above the exhaust pipe, then the fumes could work their way into the car. Rusted out frames where suspension mounts are, are definetly a safety issue. Here in NJ they test the braking and shocks and steering. If any of these are not up to par, it will definetly affect your ability to avoid an accident or control your car in case of a emergency manuver.
Personally I am not worried about those areas of my cars, it was the small amount of rust in the back that I was concerned.

__________________
1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member