O2 sensor replacement

Hi,
I've got a 98 XJ Limited. It has about 80k miles.
Should I replace the O2 sensors ? Is there a recommended replacment interval for these things, or should a person just leave it alone 'till something goes wrong ?
Thanks for your help.

Three reasons why I'd change an O2 sensor:
One-engine won't stay in "closed loop" operation (O2 failure).
Two-unexplained drop in fuel mileage ("lazy" O2 sensor).
Three-failed SMOG test, with high mileage (>100,000miles).

If it ain't broke :dunno:

For what it's worth the O2 on my 98 went well over 150,000 miles.