I am not a fan of updating the BIOS (or other HW drivers) just because a new one is out there. Not unless the new BIOS update addresses a problem I am having, a security issue (rare) or because the existing one is corrupt (also rare). I note most updates are to add support for new CPUs and other brand new hardware released after the motherboard was released.
The exception to that rule is with brand new motherboards. As Drew points out, and this is especially true with new models, a brand new motherboard may not be up to date - or, as often happens, once 1000s hit the consumer market and are placed in scenarios and used with equipment not encountered during in-house testing, incompatibilities or problems are discovered, and fixed.
The other exception might be with graphics card drivers, which often are released to support some new feature on some new game - but that's for a different discussion.
So I agree with Drew here. When getting a new motherboard, check the BIOS to make sure it is current. But after that, if your system is working fine, you probably never need to update it again.
And part of the reason to leave it alone after that is updating the BIOS is a very risky process. While the BIOS makers in recent years have made the process much easier and reliable, if something unexpected happens during the flash process (power outage or PSU failure, for example) you could end up with a paper weight.