The problem with ignorng the stacks when they are high is that if the fire gets aimed across where the tank is standing and they have a high number of stacks, they can be insta gibbed before they have a chance to react/move. You don't need to religiously dispell, but you do need to keep the stacks from getting out of hand. Also, as the stacks clear in their entirety from a single dispel, i'm not sure how you can claim it to be a waste of mana.