Looking at some random 1989 Zenith 386SX bios written in assembly so purely programmer preferences:

8 'sub al, al', 14 'sub ah, ah', 3 'sub ax, ax'

26 'xor al, al', 43 'xor ah, ah', 3 'xor ax, ax'

edit: checked a 2010 bios and not a single 'sub x, x'