question on air pressure in bike tires?

Put the max indicated and that's all.

http://www.michelinbicycletire.com/michelinbicycle/index.cfm?event=airpressure.view

1lb < POP

I'm a lighter guy and run 100psi. The LBS told me when it gets abouve 85f and I'm riding on blacktop not to go above 95psi.

Not speaking on personal authority, but it is my understanding that tires can usually run much higher psi without a safety issue... performance is another story. 

As for performance a tire's ideal pressure will vary based on the load it is supporting and the conditions it is rolling on.  There is NO one size fits all. In your situation (at 320) your tires will need more air pressure than mine (at 140) for optimal performance, that is a simple fact.

Remember, tire companies have lawyers and insurance companies that figure into every decision. Don't think for a second that if your tire says max 130psi that its going to blow to shreds if you pump it to 135.

At your size...  Max it out.  At MY size, I max it out.  I'm 6'7" and 225.  There is a chart, based on rider weight, what the tire pressure should be.  But anything over 200 lbs?  It doesn't matter, and you max them.

I typically inflate/check before EVERY ride to make sure I got my 120psi.  It may even go up some while riding due to heat from the pavement, but that's ok.  To clarify, I ride 23's...

As a side note, everyone should be doing this anyway, but I'm especially aware of not hitting potholes or rocks at my weight, just to make sure to avoid pinch flats or sidewall cuts.

You also have the option to run 25's or 32's.  Bigger tires so bigger contact patch so you can theorhetically run lower pressures.  You still want to max them out though at your size.

For example, on my MTB, I run 45 psi on my 26" wheels, and 35 psi on my 29er.