Are oxygen sensors a maintenance item, like spark plugs?

[Honus]

The service manual on my 1999 BMW 528i says to replace the oxygen sensors every 100,000 miles. Is that customary? I always thought you replace those when they went bad.

By the way, the BMW dealer quoted me about $330 per sensor. NAPA and Carquest sell them for about $65 a piece. They both sell Bosch, which is the same brand the dealer sells. My car has 4 sensors, so that is quite a difference.