If there are large periods of time where you don't have to heal or do anything, please DPS. Dealing some damage instead of healing the tank even when he's almost full HP is more recommended, otherwise you're just being lazy. Why stand there doing nothing or overhealing when you can contribute to the overall damage? I've played all 3 roles (even though my CNJ is only 27) and I've always tried to DPS as much as I could. That means optimizing my damage/rotation as a DPS, using DPS stance as a tank on bosses and using DPS skills when I don't have to heal as a healer. It's not that I like it, but why not do it if it saves you time?