As mentioned, there might be other factors at play causing fishes death...
If other fish are dying, and lets say the dead puffers body has been picked at, then yes there is a chance said fish are getting killed by the toxins in the puffer fishes body...
But from what i've experienced and read it doesnt seem many people have actually gone through a puffer fish / box fish or even a cow fish nuking their tank... with the later being one often said to nuke a S/W tank pretty quickly if they die...
Also, unless you can either test for TTX in the water, or maybe do an autopsy on most of the dead puffers, it would be hard to pin point excatly what killed them...
Just look at any wild caught stock at an LFS, chances are a lot will die before even making it to the display tanks for sale, and those that do make it, still have to battle probably getting settled into their 2nd or 3rd type of water parameters before being moved off to their next home with us...
All these factors added together can really stress out a puffer to the point of deatg, since they are a scaless fish sensitive to water parameters, and don't appreciate being crowded together for any extended period of time...