Whenever nobody needs healing, try to dps.
Play it safe at first, see that everyone looks healthy enough and won't die within the next few GCDs and try to dps every now and then. As you get better you'll get a better feel for how much damage people take and when they are safe, regardless of current HP. Someone can be perfectly safe at 20% HP and someone can be in danger at 70%.
It's not important how much HP everyone has but rather how much damage they are going to take. And dead mobs can't hit anything, so see it as a more aggressive way of "healing".

Once you unlock Holy at 45 I strongly recommend dpsing at least for the first couple of casts because Holy stuns and stunned mobs can't hit the tank.