To the OP: Reiterating what others have already said, no, you won't lose the BIOS. You will lose any customized BIOS settings and the time/date, but the BIOS itself is stored in non-volatile memory and will remain intact even if the battery dies.
I've ruined a few mobos in my life flashing the bios just for the sake of having the latest bios. Avoid flashing your bios unless you can meet these two conditions.
1. You need to flash it because you are having problems that a stated bios upgrade would fix.
Unfortunately, the descriptions of the BIOS updates posted by mobo vendors frequently don't give enough detail to determine this. But yeah, in principle I agree -- don't update the BIOS just for the sake of updating the BIOS.
2. Be financially able to replace the motherboard in case the bios flash fails.
Or buy motherboards with socketed (user replaceable) BIOS chips. This is one reason (of several) that I prefer Asus motherboards, and stopped buying MSI (MSI typically solders their BIOS chips to the motherboard, making it very difficult to do a DIY BIOS chip replacement).
It is a bad idea to flash your BIOS if you suspect that you've got bad RAM, or if the system is experiencing serious instabilities. A corrupted BIOS update due to flaky hardware will brick the motherboard.
If you have a UPS, use it. Losing power during a BIOS flash is pretty much guaranteed to brick the motherboard.
The years just pass like trains. I wave, but they don't slow down.
-- Steven Wilson