Most systems have USB keyboard enabled by default these days, i have even encounted a couple of prebuilts that dont even have a PS/2 port on the board, but like Treefrog i keep a PS/2 keyboard and mouse with me just in case i need to deal with an old system or something goes really bad.
Most modern computers nowadays the USB keyboard will work in the BIOS without changing any settings. If you are unlucky some require a BIOS upgrade to get this to work and some older computers require that you enable legacy support in the BIOS (with a PS2 keyboard). Some very old computers will not work at all in the BIOS with a USB keyboard. I find that it is a good idea to have a PS2 keyboard to hand when I come across a computer where the USB one does not work in the BIOS.