Hi OD,
Tire pressure should
always be set based on load. (Caveat; This applies to on-road driving. For low-speed/off-road there may be occasions when you would choose to adjust the pressure to suit the driving conditions)
Your tire manufacturer should be able to provide an inflation chart for your specific tires.
You can then weigh your axles and adjust the pressure according to the chart.
Here is a sample of the data that Michelin provides...

As others have mentioned, pressure should be checked when the tires are cold. Do not deflate or adjust pressure when the tires are hot.
Cheers,
-Mark