I use the placard recommendations, especially if I have the same size and type of tire on the vehicle as stock.
On my truck, I did some homework when I switched tire sizes and went from a P-series tire to an LT Load Range C tire with a Max inflation pressure of 50 psi. I actually figured what the derated load carrying capacity of the OEM tires were (remember that P-series tires must have the their weight rating derated approximately 10% when used in light truck applications), then pulled the Bridgestone Light Truck Tire guide, and matched pressures. The OEM for my truck was 35f/35r, while with the new size and LT series tire, the same weight is carried at 38f/38r. 25,000 miles later and no wear problems to report...
On my truck, I did some homework when I switched tire sizes and went from a P-series tire to an LT Load Range C tire with a Max inflation pressure of 50 psi. I actually figured what the derated load carrying capacity of the OEM tires were (remember that P-series tires must have the their weight rating derated approximately 10% when used in light truck applications), then pulled the Bridgestone Light Truck Tire guide, and matched pressures. The OEM for my truck was 35f/35r, while with the new size and LT series tire, the same weight is carried at 38f/38r. 25,000 miles later and no wear problems to report...