No, I've seen firmware updates fail without having a power loss. However, that is the main cause of failures.
clarkjd was exactly correct in that BIOS do not need to be updated unless you are having issues with the computer. This is not a reworked driver for your video card that will jump your computer in performance. BIOS is all about stability and compatibility, if you are not having issues with that, leave it alone.
With all that said, the failure rates of BIOS flashes aren't very high as long as you follow the instructions. If you feel you need (not want) to do an update, I'd say do it. Please be aware that you may end up needing to replace the mainboard if it fails to take properly.