From what I've read (various cast and crew quotes, etc.), it seems as though the permawhite vs. makeup thing has become a moot point.
Why? I found one of Nolan's quotes to be particularly telling - he states that Joker's face has become like a permanent "Francis Bacon portrait," which makes perfect sense (just Google Francis Bacon.)
In other words, Nolan's/Heath's Joker treats his face much like a canvas - one that is never truly wiped away. Despite the fact that we've seen many shots with Joker having varying states of makeup - clean, greasy, smeared, etc. - I don't think we'll ever see him without it on.
In other words, the makeup might as well be permawhite. It's as though he's used strong enough material (not some cheap store-bought makeup) and left it on long enough that it's almost impossible to wipe away completely - kind of like that stubborn spaghetti stain on a white shirt. He's been like this for years and has accepted it as part of his "face," treating the makeup clown face as his own. I can actually see a conversation taking place between Gordon and some officer during the incarceration scene, with Gordon asking "Why haven't you washed his makeup off?" and the officer replying, "It won't come off!"
So my point is that, while technically it isn't permawhite, it's being treated as such. Actually, it's all the more horrifying, if you ask me: the Joker's face can become more and more ghoulish depending on the situation - streaking and smearing and then appearing clean again in a later scene - whereas a permawhite Joker would have the same face no matter what the circumstance. It gets treated almost like an evolving prop.