Pretty much what most have said..

Being very VERY comfortable with my WHM, I know how much I'm healing for, vs the incoming damage to know how much more the tank can take.. so if I know there's some time available I'll try to throw some rocks at the enemy. I do understand the feelings from the tank's POV, especially if we're both strangers to each other.. so I probably just pop back to healing at around 50% of tank's HP since while I also want to make the run go faster, I also understand that throwing an additional rock isn't going to matter much and I'd rather be safer than sorry.

That's the general gist of it I guess, if some healers let the tank HP go down even lower, it just means they're more confident of their healing and want to truly maximise their damage. There are those flop ups by healers that do almost no healing (god knows why).. and that's pretty much a given that the healer went too far (or maybe it was the first pull and tank's HP dropped unexpectedly fast, but that's another topic)