On a road bike, the greatest stress a headset takes is being turned 10 degrees every minute or so. The bottom bracket spins constantly at 70 to 100 rpm under torque. I service my headsets every once in awhile, just so I can say I did it if I sell the bike, but I have a hard time sinking a lot of money into a headset, considering how little stress it gets compared to the rest of the parts. Clearly this doesn't apply to bmx bikes or bikes that are actually raced.