I believe that companies will only void the warranty if you use an unsupported bios update ( from a 3rdParty developer, or from another region ). I've never heard of a company releasing a software update for their product and then claiming that if you updated, they will no longer honour their warranty.
In any case, Asus does not consider updating the bios to void your warranty.
However, that being said, if the update of a bios messes up WHILE its updating ( the machine turns off) , then you may be up a creek...