Brad,
My plugs and PCV valve usually look pretty good when I replace them, but I replace them anyway. I'm not saying that you'll have problems, or you'd run mega rich or anything, I'm just saying that the O2 sensors are probably the most critical sensor for your car to run perfectly. Replacing them at regular intervals will ensure optimal performance for you car (gas mileage especially), just like replacing plugs that don't really need to be replaced.
Do what you think you can afford to do.
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351W 89 Mustang GT Convertible