Dell has posted this information for several years:
If you are not experiencing problems on your computer and are not aware of any additional features in the BIOS that are needed, then you may not need to perform the update.
If a BIOS update is not performed properly, it can actually harm your system. It is best to review the release notes to determine whether the update will benefit your system. Also, review the instructions that are included with the BIOS update software, prior to performing an update.
Even though this is from Dell, I believe most (if not all) computer manufacturers follow the same logic. To put it another way, "If it ain't broke, don't fix it."