Healing a pet generates way less aggro than healing a player character. You should take advantage of that and inform your priest buddies if they aren't aware of it.
We have no numbers or 'scientific' proofs, but it seems that healing a pet generates only a low initial assist aggro. Any direct attack from the pet will override that and you can heal the pet at will without ever getting aggro.
This is important to understand when your pet tanks multiple mobs :
- If your pet has attacked only mob A, healing it will get you aggro on mob B,C,D... who were assisting mob A.
- If you have rotated, even just for a couple of swings or pet procs, between mobs A, B, C, D..., you can cast as many healing spells on your pet as you want (try with a cleric friend chain Complete Healing a pet, you'll see) and all mobs will stay on your pet.
In short : when your pet tanks multiple targets, spend the first seconds of the fight rotating between targets to be sure pet has hit every mob before casting healing spells. Once pet has hit every opponent, healing is at will for yourself and for priest classes.