I just create a directory called bios and add it to my mame.ini file (eg. so it looks inside the roms folder and roms\bios) that way I don't have to even try hiding it, etc.
It works great and it keeps my main roms folder uncluttered.
I also have roms\parents where I place parent roms that I don't want to see in my games list but are required to play the clones, etc.