Checking for under-inflation is such an important step that it cannot be overlooked. In fact, a tire that has been operated below 90% of itʼs rated pressure for any length of time should be considered for replacement. This is because there may be internal damage that is not apparent from a casual observation.
Tire inflation should be checked before each flight because the loss of pressure can be as much as 5% per day in normal operation in certain situations. Tires exceeding this normal rate should be checked for abnormal defects. In addition, changes in temperature will have a significant impact upon pressure during a single flight from a very warm to a very cold environment.
The recommended rule is to fill the tire to optimal pressure based upon the coldest temperature that will be experience. Another good rule of thumb is to initially fill the tire with air to 105% of the recommended pressure.
To insure compliance, the POH of the aircraft you are flying should provide you with the proper air pressure figures. Using this data, tires should be checked with a gauge prior to each flight. This is especially true for aircraft that are equipped with wheel covers since direct access to the stems are sometimes difficult to achieve.
For those who wish to learn more about proper tire inflation issues, the FAA has an excellent course #ALC-269, The Importance of Tire Maintenance on Aircraft Safety.