Consumer Reports says every month is exactly how often you should be checking your tires. Consumer Reports also says that years ago you could see a tire going flat, but not anymore. Newer more rigid tires often don't show clear signs of being underinflated. Another common mistake is not using the right tire pressure. You can usually find it right on the driver's door.
Overinflated tires cause poor handling and uneven wear.
Make sure you check the tread every month, too. Take a penny. Place Lincoln's head in head first into the tread gap. If his head is buried inside the groove, then there is sufficient tread left. On a bald tire, the penny check looks entirely different.