if the caliper on the side not being replaced seems good, and the caliper slides free, fine. for me, with 80% of braking force on the fronts i say replace it. as a former road racer i would always replace both calipers. as i consider brakes the most important thing on a car, racing or not. i am not willing to take a chance on old vs new. i have, in the old days replaced just one wheel cylinder thou. but calipers never will.
i always cover the pads with anti squeal juice.