I bought a 1997 mustang 14 years ago. It had ming done to it some time before I bought the car(the ming package also coated the bottom of the doors and inside the rockers etc). I had seen some Mustangs (back then) of the time period pretty rusty to the point of rusted through. When I bought the car, the car was very clean in terms of rust and when i sold it 3.5 years ago it still looked very new and rust free. One of the more sold mustangs of the era.
Just thinking about rust proofing that oily stuff that never dries would probably be best used inside of frame rails/unit body frame channels and inside the bottoms of doors, inside rocker panels inside rear 1/4 panels. Those are the arias I would rust proof before undercoating the visible stuff. for the visible stuff (including floor pans, wheel arch lips, bottom pinch section of the doors make sure in the winter months you wash the car regularly. When I wash my cars I all ways wash the floor boards as well as get in and wash all the salt and mud out of the wheel arch lips, in the gaps of the plastic moldings and open the doors and low pressure wash the bottom edge. I think keeping the car clean when possible is very good for keeping it long term. My 1973 celica had rust coming from inside the front sub frame rail working its way out. So coating the inside hidden places is probably most valuable.