Pets are low priority, and pet classes can use abilities to heal them, but if you aren't doing it out of laziness then there's a problem. Healing your pet is a DPS loss for any class that uses a pet.
Hunters have it the easiest with Mend Pet.
Warlocks have to channel a spell, and have defensive cooldowns that drain 50% of their pet's health, Health Funnel is a big DPS loss.
Frost Mages have to cast frost bolt at their pet to heal it.
Death Knights can summon a new one so frequently it doesn't matter as much.