Ok pet peeve of mine here.
I find one of the most common mistakes healers do is trying to top up everyone, often at the expense of being on the GCD when the tank takes a huge spike.
When healing challenging content I use this as a personal rule: DPS can wait. If I have swiftcast ready then I'll heal DPS, because if the tank spikes I know I can cover it with a swiftcasted Cure II. It's situational of course, if I know there's no big spikes coming on a tank I'll happily heal the DPS meantime, but if there is any doubt, DPS can wait.