In any case I always try to find and retrieve a dead fish, but it really depends on the tank size and age of the tank. A large tank with established bacteria would most likely handle that with no problem. Sometimes, its physically impossible or not worth the time to rip apart the aquascape. However, if it is a small or new tank, I would definitely try to get it out so the water quality will stay consistent.
If you can't find him or it's too difficult to rip everything apart, then it should be okay. You might get a bacteria bloom though. I've never personally had a puffer poison a tank.
If it were a boxfish though, then you'd be in some trouble[emoji23]