If the plugs are ok, and fairly new, there is no reason to replace them. When I changed my plugs, I bought 3 coils just in case. I ended up not installing any of them because all the factory ones were fine. 39,000 miles. I do use a lot of oil, fuel additives and valve cleaner spray regularly - maybe its snake oil, maybe its not!

 

Panamera is directly injected, which means fuel is injected into the cylinder below valves. In non DI cars, fuel is injected into cylinders from above valves, which causes the valves to be cleaned and washed by the fuel itself. Then, additives did help to keep them clean and clear. In DI case, the washing is not happening, so the valves end up gunking up over time, necessitating walnut cleaning scenario. I do not know what dictates how quick the gunk accumulates or not.