How to determine if it's safe to overwrite global Blizzard function

[Ethly]

I want to rewrite global functions LocalizeFrames and CombatFeedback_OnCombatEvent. I can do just that:

function LocalizeFrames()
	...
end

function CombatFeedback_OnCombatEvent(self, event, flags, amount, type)
	...
end

And from quick testing it seems to work. However I've heard about concept of tainting. I can achieve the same result with hooksecurefunc, but it causes unnecessary code to run (I'll overwrite Blizzard code, but it'll be called anyway). I don't know exactly how tainting works. So the question is: is it safe to overwrite those functions and how can I tell in general, which functions are safe to overwrite and which functions must be hooked?