regular street: 36
long touring trips: 42
road course race track: 30
Good rule of thumb!
However, there is a wide variety of tire pressures reccommended by different manufacturers. Most tires will function just fine at any pressure between 25 and 42 psi. A good rule of thumb is the higher the pressure the cooler the tire will run and the longer it will last. Also the higher the pressure the faster your top end speed as well as accelleration as low pressures rob horse power.
On the flip side, the lower the pressure the quicker the tire heats up, more traction due to the additional heat (To a certain point) the hotter the tire runs the shorter the life of the tire, and the story goes on.
Tire pressure should be adjusted to serve the purpose in which you are trying to accomplish. Myself, I usually run lower pressures than the average guy as I'm not into high speed and most of my rides are at 100 mph or less. I spend a great deal of time on the rear tire. If I'm going out to spend the afternoon with the front tire high in the air, I only run 25 psi cold in the rear. I never do high speed at that pressure.
Tire pressure is an individual decision that each rider has to make for himself. If you want good tire life run 42 psi. If you heat your tire well before you lean your busa over, you can drag the plastic with no problem at 42 psi if you have good technique. The down side is, while dragging the plastic with 42 psi the margin of error is more narrow than if you have 30 psi.
For most of us, tire pressure is not a big issue as the average rider never drags his foot pegs much less his plastic so whatever tire pressure you choose, you'll be just fine. If you are a plastic dragging maniac, you won't be asking about tire pressure. You'll already be way past that stage.