The rear does not do much work, depending on how you drive.
That said, there's no good reason to not replace them, besides being cheap.
At some point soon, the slave cylinders will be leaking (if they aren't already).
It would be a good idea to replace them, the drums, and the shoes at some point. You could even get a more grippy shoe and surprise yourself with how well you stop.
(Another point, if you feel your car nosing a lot while stopping, it might be a good time)