How often should the headset be tightened? I live in an area with a lot of potholes and commute daily (between 10-20 miles depending) on a modern road bike. Today while coming in hard (18+mph) to a stop, I went over a worn paving layer with the front heavily loaded. I heard a loud CLACK, and my first thought was to look to the carbon fork. No cracks, but it got my mind on to my head set and head tube. (It was probably just the rear derailleur swinging out and coming down on the B screw.)

What's the minimum mileage interval for headset tightening, and what are the symptoms that indicate it's time? (It's not covered in the Optimal Maintenance question.)
To check your headset, sit on the bike, lock the front brakes, and push forward/backward with your feet. If you can see any looseness in the headset it needs to be tightened.

Since it's so easy to check one should check a few times a year. But no real "schedule" is needed.

I've seen headsets that were remarkably loose.  Though looseness affects handling to a degree, most people don't notice the problem until (in the old threaded headsets) it gets so loose the ball bearings fall out.  Then the bearing grinds and makes handling difficult.