There is always a risk involved when messing around with your Bios.
Chances are, warranty will be void if you make a mistake and can not get old Bios back.
But, there are some good reasons to change or modify Bios.
Bios can be somewhat like drivers, in that a later version may correct issues.
Normally, a Bios is keyed to a specific manufacturer.
Some drivers will only work with certain manufacturers cards, so modifying a Bios can get around this.
You can also adjust the Bios you currently have, to change speeds and enable settings.
The following program allowed me to change the core and memory on my Visiontek GF3 Ti200.
NVIDIA BIOS Editor
It was 175/400, and now it is 200/500. Nothing else was needed, (such as Coolbits or Rivatuner.)
Before flashing Bios, always understand that you may have to restore old Bios blindly using only a boot disk.
I have never had a failed Bios flash, and I test all the time. But, it could happen next time. Just be warned.
EDIT:
If you are modifying Bios for things such as core and memory speed, first confirm your system can handle
the settings by running patches or software like Rivatuner for a while. You don't need to modify bios to
get speed increase. I have just had some minor compatability issues using coolbits or Riva tuner, and they
have now been resolved by simply adjusting Bios to speed I prefer.