To which I must respond, quite possibly, Never.
Tossing lightly aside the issue of delivering preachment when you promised (and were paid for by) the reader entertainment, is the world really going to be a worse place for your not preaching anywhere, anywhen?
Much of what I said about sagacity in other posts applies here. Indeed, come to think of it, they often are about preaching. Particularly about the chance to show off the depths of your shallowness. Then, you can also do it by producing flat characters to go through the motions and have them display alternate sides. And they have to be flat. Honest portrayals are perfectly capable of producing works that your opponents will think are pro-their side.
The worst I have seen recently is writers who demonstrate that they have either not read Plato's Apology or not fathomed the reason why Socrates is the wisest man in the world. They are the ones who decide to introduce a topical theme into, say, a zany webcomic. They fail to realize that the ability to write a goofy webcomic does not translate into particular insight into a current political situation.
Meanwhile, there is the little matter that you do not have to go to insert meaning.
Let the pictures tell you their own moral. For the moral inherent in them will rise from whatever spiritual roots you have succeeded in striking during the whole course of your life. But if they don't show you any moral, don't put one in. For the moral you put in is likely to be a platitude, or even a falsehood, skimmed from the surface of your consciousness.
When it does arise, it does arise. Sometimes you even have to have characters talk about the philosophical significance of the events they are living through. But even when the plot demands it, it's wise to be wary. The higher the theme rises, the more catastrophic for it the fall if it fails.