In that case, you should consider replacing the rear O2 sensor.
I'm not a huge fan of replacing both "just because they're old", but it'* really your call and what you are comfortable with. Your PCM is pretty sensitive and will usually let you know when you have a problem.
On the other hand, I also prefer to repair things that keep the SES light on. Ignoring them allows other things to go wrong with no indication to you. Use the SES/CEL as a tool as it was designed, especially since you have no scanner (or really a need for one).
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s