I would only heal tanks if the damage dealers are down or actually need healing, tanks are supposed to be able to take hits(and don't need much healing, reinhardt has his shield, winston his barrier, roadhog can heal himself, zarya has the particle barrier and 50% of her health consists of shields), while most of the damage dealers are squishy so just alternate between healing and damage boost on them can be devastating, especially on pharah and junkrat, even zarya at full charge(90 damage from the secondary fire +30%dmg boost = 120dmg or so per shot)