Flashing the BIOS means updating it, and there's no reason to do that if there's no problem on your system, and if there's no update issued for any problem you might have.
Interruption to power while flashing can render your system useless - it is not a task to be taken lightly. Not even a technician will guarantee you that a BIOS update will be performed with out problem.
Some enthusiasts update BIOS with alternative versions designed to let the components of the PC interact with each other quicker, but again, there is risk involved.
Again, I'll stress - a BIOS update should be performed ONLY if there is a hardware problem which a BIOS update has been released to corect.