the risks are non existant for tweaking your existing bios because there is always a method of resetting your bios on the motherboard, although ive never had to do this yet!! most modern boards have a pair of jumper pins you need to bridge for around 5 seconds. older, more primitive boards may require removal of their onboard battery for 1/2 an hour to acheive the same effect (1/2 an hr because the on board capacitors can hold a charge this long).

a small but serious risk is apparent when you "flash" your bios to the newer version. this process takes up to 30 seconds and you get the option to save your old one when you do this.
the risk i speak of is an interruption to the power supply when you are doing the "flashing". if you use an uninterruptable power supply then there is no risk.