Depends on the tire too. Is it the same tire that the factory put on? Some tires use more air pressure to support the same weight, in which case you would have to put in that extra pressure if you are supporting that much weight.
Here's what I was told: Find out how much weight is on the wheel (weigh it somehow) for your normal riding conditions and add a bit onto that for when you have a few too many burritos for lunch. Read on the sidewall of the tire what it's maximum weight carrying capacity is at full pressure. Calculate what percentage of that maximum capacity the wheel is actually carrying (from when you weighed it) and multiply that percentage times the maximum pressure written on the sidewall. The result is the pressure you should be running. If you increase what you are carrying like loading up for a trip or riding two up for any distance, increase pressure accordingly. At least that's how it was explained to me by a guy that started riding around the time God was born.
1987 Softail Custom, stock except for added chrome, and still running like new after 100,000 miles.