Had my '04 undercoated, never did take it in for yearly rust inspection/touch up. I shoot the salt out from underneath and get the wheel wells at the car wash really good a few times each winter so mine's pretty clean.
If it's not visible just clean it up with a wire wheel then spray it with rustoleum front & back and put the cladding back on. I rough up my hitch reciever, bumper guards and mounting brackets with a wire brush and sandpaper then spray them with flat black rustoleum each spring. It inhibits existing rust and seals out moisture, an ounce of protection...