Tire pressures can be a tricky thing - and it's different on each set of tires, vehicle, suspension setup, alignment, etc..... The biggest thing is to watch the wear closely and do regular rotations to ensure even wear across the tires. When towing heavy loads the tires need to be at higher pressures as the max capacity of the tire is at the maximum pressure.

There are methods to gain the correct psi for your particular application - do a search on 'chalk test' on Google. One of the best explanations I've seen is here:

Checking Tire Pressure - Determine Correct Pressure