The pressures on the tires are for maximum cold pressure. The pressure will increase as the tires heat up. Usually, the heavier the load the tire is carrying, the higher that the pressure should be. So, if a specification for a particular bike with max load called for max pressure in the tire (we'll use 42 psi for an example) you would fill it to 42 psi when the tire is cold. When it heats up, the pressure will increase to above the maximum recommended. This is factored in to the tire design, so you would not adjust the pressure at this time.
Now back to the question about how much pressure in your tires. Go with the manufacturers recommendation (different for SM or BA) for starters, then you can make slight adjustments (up, never go below). I run 38 psi in the Metzelers on my Speedmaster, the recommendation is 36.
