Every one I've ever seen goes:
1)Floppy
2)HDD
3)CDrom
4)Other,
but you're welcome to change it as you see fit. If you don't (or rarely) use a floppy, set it to boot from the HDD first. That should save a little time during boot-up. If you ever need to boot from a floppy just change it in the BIOS first. You can also set it to boot from the CDrom first if you're using a bootable CD.