Healing aggro is discussed here.
"Yes, healing is one for one, *however*-
Healing requires an intelligence check on the part of the mob to notice it at all.
"(Note that some mobs are coded to have a hate modifier regarding heals, but these inconsistent in the game world and not something you normally need to worry about)" was written for tanks- obviously, it is more pertinent for priests.
Some mob types are less likely to notice a heal- this includes golems, clockwork, and undead.
The easiest way to demonstrate this is to watch your "in/out of combat" box. Even with zero Spell Casting Subtlety, sometimes, you will not be put in combat for casting a heal. This was because the mob didn't notice you.
Group heals are far more likely to be noticed, and generate more hate in general, because it forces a check for every person or pet that it hits, which can be anything from one (if you're using it while soloing for some reason or other), up to twelve (if you have a full group with pets) or more. Splash has a fixed hate amount so that you don't implode using it, but it too, is near impossible to stay off aggro using, due to the number of checks."
Of course, the most non-classic aggro mechanic on p99 is w/r/t Undead. On live classic, undead attacked the closest person. Period.