A BIOS update is more of an "as needed" consideration.
Examples of when a BIOS upgrade is needed are:
1) new CPU that's not supported by the current BIOS
2) new RAM that's not supported by the current BIOS
3) serious problems that are specifically related to the BIOS (CPU suddenly "underclocks," RAM timings changed, CMOS settings won't retain (that's usually a CMOS battery problem, but can also be a BIOS problem).
Other that the above, BIOS upgrades shouldn't be applied liberally. One false move and, as Patio said, it can become an expensive paperweight...