You're right RShara, there is more in BoM I forgot.
This is the bit I think:
"Sensation— and agony— flooded back into him as his metal-mind healed his spine, focusing on the worst wounds first."
He then heals legs, shoulder and toes are mentioned as well.
From the phrasing it's not clear if this is a concious choice or not. I like your idea, Belzedar, that a very experienced bloodmaker could overrule this: perhaps preferring to heal a lesser wound on the face say, purely for cosmetic reasons over something slightly more serious.