It is usually a "safe" operation if the following is true:
1) You have an issue that you believe a BIOS update may help with (sometimes this is as simple as system stability)
2) The machine is powered up and preferably running on a fully charged battery if it's a laptop or UPS if it's a desktop (you really don't want the power going out while your BIOS is flashing)
3) You are 100% sure you have the correct BIOS update file (usually the flashing program checks, but never hurts to double check!)
4) You have read the instructions and understand how to use your motherboards BIOS flashing utility
I always keep my BIOS up to date as part of maintenance, some people use the argument "if it ain't broke don't fix it", but with gaming rigs with high end components BIOS updates are frequently linked to performance and stability, so I do them whenever I see one available (my personal preference).
If you need help with any of the above, don't hesitate to ask, but include your Alienware Model info.