Is dodge rating or parry rating a more preferable for tanking if you are mainly focused on making healing easier for the healer

Both of them are roughly equal as a tank.
Paladins and Warriors will want to get more mastery, and then a mix of Parry/Dodge, while Druids and Death Knights, generally go for dodge/parry.

Anyways, in conclusion, dodge and parry are both EQUAL in terms of making it easier for the healer, although you should stay about even (10% dodge, 10% parry, etc.)