The more frequently you do them, the better the chances of saving a slow leaker.
Tire life decreases exponentially with underinflation. According to TMC's RP 235A - Tire Inflation Pressure Maintenance, running a tire 20% underinflated shortens its life by about 12%. Running a tire 30% underinflated lops about 30% off its expected service life. It only gets worse as pressure drops.
TMC's RP 235A recommends using a high-quality tire gauge, calibrated regularly from a master gauge to ensure accuracy. As for the actual operating inflation pressure, users should consult the Load and Inflation tables relating to particular tire sizes and anticipated loads. Interestingly, a typical 22.5-inch low-profile tire run in dual configuration is considered properly inflated for the load carried on a 34,000-pound tandem grouping at only 75 psi. That same tire in a steer position requires 110 psi for the 12,000-pound load on a steer axle.
Overinflating tires in dual positions to 95 or 100 psi is acceptable, and it provides a margin of error for underinflation if pressure checks are not conducted regularly.
The biggest risk in dual tires is inconsistency. RP 235A notes that a pressure differential of just five psi across the tires in a dual set creates a difference in circumference between the tires of up to 5/16 of an inch, and that difference in circumference increases with a wider pressure differential. Two tires of different circumference will not cover the same distance over a given number of rotations, but because the wheels are bolted together, the tires will scrub along the road, causing rapid and irregular wear.
From the March 2012 issue of HDT