Any rust prevention measures are a good idea regardless of how long you keep the car because it will be easier to sell a car in good condition to a private buyer than it will a Rusty one .
NoIs it worth while on the new shape panda to spray inside the sills and paint the front subframe with stone chip?
There are still parts of the car which are prone to premature failure due to corrosion, but the emphasis has shifted from the bodyshell to the components.
On the Panda, think sump, springs, spring pockets on rear subframe, handbrake cable attachment brackets, rear brake drums etc.
I think we've moved on from spraying wax into box sections, but a little attention to the known weak spots is worthwhile if you plan on keeping the car.
The real enemy is damp mud holding water in places it shouldn't sitThere are still parts of the car which are prone to premature failure due to corrosion... spring pockets on rear subframe