Good tanks throw Guard on whomever is likely to pull agro. That usually isn't the healer (assuming the DPS are following the kill order).
Clueless tanks throw Guard on the healer, no matter what.
Bad tanks don't use Guard at all.
I did Athiss with my PT a while ago at level 19 and there was level 22 Marauder in the group. I didn't have to think who to Guard.