So far I have fluid filmed 3 vehicles, Amsoil HDMP'd 1 and CorrosionX HD'd 1 other. The winters here are very harsh and the salt/sand mixture on the roads is horrendus. I would say rinising the undercarriage with high pressure from a regular garden wand/sprayer is enough to remove most of the salt and provide at least some benefit, without removing your fluid film or other treatment.
If you have a pressure washer, set the nozzle to a less powerful fan spray and hold the wand a little further away. That method of rinsing will also remove most of the salt without hurting your fluid film or other coating.
In the spring time, thoroughly blast the undercarriage with high pressure from a pressure washer (use soap/salt away if you want) and let your car dry. After the undercarriage is dry, apply another heavy rust proofing treatment of your choice and you will be covered for another year.