[95Viper]

Bios updates... it depends.
Does the update improve compatibility, update option rom(lan, raid controller,etc.), improve performance, correct an issue you may have?
Then, yes.
If not, then maybe. If your system is running fine... again, maybe.

Sabertooth 55i 2003 BIOS
1. Improve memory compatibility
2. Improve system stability
3. Update CPU Level up function
4. Update Marvell Option ROM
5. Support new CPUs

IMO, if it were me, I would for stability improvement and the Marvell ROM update (if you use the Marvell controller, I do and with the update, plus the latest drivers on my MSI MB I saw a slight increase in read and write speeds.

I never had a problem of having to re-install windows after a bios update.

But, you may have to go into the bios and put your settings (for the disks, O/C, option roms, memory timings, voltages, and etc.) back to what they were, before the update, if you tell it to re-set to defaults or, if it does it by default.

For, example, the bios may change your drives from Raid or ahci to ide.

Edit:

I have heard different things about ASUSupdate, both good and bad.
I usually use the DOS or USB method, instead of the Windows method.
Read the manual; it, usually, has info and a walk through of the different methods in it.