Dear friend,
Upgrading the EPLD is not a mandatory step after upgrading the NX-OS. The EPLD is an electronic program logic firmware, let's say, it commands how the hardware and software are communicating to each other but not obligatory to apply.
I would personally recommend undergoing an EPLD upgrade to a device that is still not in production and/or laboratory gear that is yet to be deployed. The EPLD upgrade might help in case of some weird issues that can relate to the hardware.
Thank you very much for your question, have a great day.