Tire pressures are probably one of the most familiar aspects of a race car that most people can understand easily. Within iRacing.com's sim however, there is much debate on the subject of how to use them.
For most of us, generally you have learned that you want to run a few laps on the race track, then check your tire temperatures. Most people will take the average of the inner and outer temperatures of each tire, and if the middle temperature reading is higher or lower than that number, you have either too much or too little tire pressure in that tire. The general rule of thumb is to adjust 1 psi for each 5° F difference that you desire.
It is hard to know the optimal tire pressures at any given track, or for each car and setup, but with the current tire models within iRacing.com's service, there seems to be little consequence with having improper tire pressures in terms of tire wear. Your tire pressures will create a characteristic feel that will complement your setup, but if not set correctly can spin you out or push you to the wall.