When I use the PGs, it depends on what upgrade I went for, For G43s, I go triple offensive vet so they can get a ROF and accuracy bonus. For the Mp-44s and Panzershrecks, I go triple defensive vet so they are much harder, especially with the Panzershrecks, they need to close the distance to kill tanks.
But If using vehicles, The rule of thumb is if there are lightly armored (HTs and AC) go triple defensive to better protect them from the wretched US sticky bombs and BARs. If they are along the line of tanks (Pz IV IST and Panthers) I go triple offensive vet, to improve pen and accuracy, especially with the Panthers, they need the be able to hit and kill infantry formations.
Now when it comes to hummels and mortars, I don't notice a direct affect with offensive vet, but I might help.