Depends on the tire. On some tires we run the manufacturers spec. Other ones we run pressures a lot lower than what the manufacturer says.
Tire rack has some good information to get you started:
http://www.tirerack.com/tires/tiretech/competition/racepres.htm
One problem with Viper tires is that they are huge and tend to be a little thick. Differences in the manufacturing process mean that the tires aren't even in terms of consistency.
One of the best things you can buy if you are are serious about this is a pyrometer with a small probe (about $125). Don't get one of those laser/infrared pyrometers - they just get the surface temp and you need to know the real tire temperature. You are looking for consistency across the tire. Measure the inside, middle, and outside of the tire. If the tire is hotter in the middle, that means that you have too much air pressure. If the middle is lower, that means too little.
Most tires don't end up with consistency across them. Usually you have to take a look at the direction of heat differences. Keep in mind that there is no one 'right' temperature. We maintain spreadsheets of temperatures according to air temp, driver, and track. My Dad, for example, likes to slide the car (old Sunbeam Tiger driver) and he puts about 10F more in the rears than I do. We typically lower the back right about a half pound when he runs.
Finally, keep in mind that the air you put in the tires also matters. Try to keep dry air in the tires. Moisture in the air means that the water heats up inside the tire and makes pressures go crazy. That is why airplanes often use nitrogen (inert gas). Some race cars use it too, but it's too much of pain for most of us.
Have fun!
