Stone guard varies heavily and when the fight is under control you don't need all the healers to heal (that you have unlimited mana and might as well keep spamming to avoid silly deaths is a different matter).
WotE hc. While the tanks are dancing one healer can easily handle the titan gas damage on their own, depending on your setup/if it's done properly you should have little damage apart from this. Considering the lenght of the fight and the amount of tank healing (=mana consuming) I can't even imagine it being possible to heal every single moment if you're geared properly (not overstacking spirit combined with the awful thing called spirit flask).
Garalon hc. Even if the healing required is quite high during the entire fight you obviously have less (no crush, low stack) and more intense (e.g. crush going out) parts of the fight, during the fights with low/medium intensity there's no need for every healer to spam to keep the raid up.
Empress hc. The only fight where I'd argue that you might need to heal constantly, even then the amount of healing you have to put out varies based on the phase (p3 def. doesn't seem to require 8 healers spamming to get through it, parts of p2 also seems less intense).
Tsulong hc. Dread shadow stacks vary and if they are low on most players all of you don't need to spamheal at that very moment. In day phase you don't 'need to' heal outside of breath/light, that you might as well due to the added regen is another matter.