Most definitions of litotes are hard to understand, even for a rhetorician like me, who is supposed to have a handle on this particular trope.
You are not off-point [sorry!] when you ask if litotes is just for emphasis, and if it can also be a way of hedging. It's a little of both, but more often than not it is used for emphasis, even if it sounds like hedging. In other words, all instances of true litotes (not the "false" litotes in the second example you provided) involve emphasis, but the subset of the hedging type of litotes is part hedging and part emphasizing. Makes sense, right?! Of course not.
The "back-handed compliment" aspect of litotes (mentioned by someone above), could be interpreted as hedging, in that the writer/speaker is not clear in her approbation, similar to the soup taster you allude to. Why could not the soup taster say simply, "This is pretty great soup"? Because he wants to draw your attention to what he is saying by using a relatively rare method of saying it. In other words, he's saying, "Look how I'm using words [this is the emphasis part]. The subtext could be, "Look at how clever I am to be talking this way," in which case listeners might think him egotistical.
Our soup taster is engaging in a not-so-subtle form of persuasion by interpreting his reality for you in a slightly skewed way. Might a fellow soup taster say to Mr. Ironic, "I disagree with you; this is great soup!"? Yes. Who is to say which evaluation of the soup is more accurate? That gets us not only into the notion of how we describe our reality for others, but also the notion of taste, of which the Romans said, "De gustibus non est disputandum," and which is off point here.
The following are a few examples of "hedging-," emphasizing-, and indeterminate types of litotes taken from Robert A. Harris's "A Handbook of Rhetorical Devices." Google it; it's online:
"We saw him throw the buckets of paint at his canvas in disgust, and the result did not perfectly represent his subject . . .."
Now, the speaker may in fact be emphasizing her point of view. That interpretation requires a context to make it valid, however. The nonverbal aspects of the utterance could be enlightening in this regard. If uttered in an ironic tone, then you have emphasis; if in a matter-of-fact tone, it's kind of emphatic in that the speaker is emphasizing the non-representational aspect of the painting, but it could also be indeterminate, albeit mildly ironic.
"In Florida, heat waves are not rare in summer."
If said in response to a mundane question such as "Are heat waves rare in Florida?" and delivered in an informative tone, then this is kind of indeterminate. The speaker is obviously not hedging, but is he emphasizing? Well, again, context and tone of voice would give us clues beyond the merely verbal and syntactical clues.
"Hitting the telephone pole certainly didn't do your car any good."
Here, the emphatic type of litotes is likely used. The word certainly indicates this is so.
(from Richard Nordquist's About.com): "He is noticeable for nothing in the world except for the markedness by which he is noticeable for nothing” (E.A. Poe).
The word markedness is a clue that Poe is emphasizing a chap's invisibility!
"He who examines his own self will not long remain ignorant of his failings."
Here, the speaker is likely using emphatic litotes in that he is contrasting the examination of self for both strengths and weaknesses, with the refusal to examine self, particularly for weaknesses and failings. This is a subject certainly worthy of emphasis. Was it Socrates who said, "Above all, know thyself"?
"Overall, the flavors of the mushrooms, herbs, and spices combine to make the dish not at all disagreeable to the palate."
Compared to your soup taster, this gourmand is likely emphasizing the goodness of the dish by drawing your attention to the unusual way in which he does so, but again, context is crucial in making a definitive "call." Her use of the words not at all seem to indicate it's an emphatic form of litotes.
"None of the flowers appeared not to have bloomed at that time."
This isn't likely litotes, just a poor way of expressing a thought that could just as easily have been worded, "Of all the flowers we planted, not one failed to bloom by April 15."