This is something I wrote for Creative Nonfiction about truth in general and truth in this genre:
Lies are everywhere. The media tell lies, strangers tell lies, politicians tell lies, the lighting in fitting rooms tell lies, as do zippers, belts, buttons, magnified mirrors, and weighing scales. They're told by everyone, and they can even come out of the mouths of the people you trust most. The truth is not always easy; in fact, it rarely is. It can hurt to hear and it can be difficult to say. However, this does not mean that it should be avoided or censored or not said. The truth needs to be told, and in the end it will always come out. I value honesty greatly, and I'd like to think I'm a pretty honest person, but there is a place in the world for lies.
If you were invited to your boss' house for dinner and he/she asked you what you thought of the lasagna, you probably have little choice but to say that it was good. The lasagna could have been horrible, it could have tasted like something you might scrape off the highway with a shovel, but would you say that to your host? Or might it be better to go for something a little less... true, and politely decline the invitation for seconds?
People ask each other how they are all the time, but they don't really expect someone to tell them the entire truth.
"How are you?"
"Fine, thanks. You?" Not: "Bad," because then you'll have to explain. "I suspect my husband is cheating on me, the dog threw up its own poop on the new carpet last night, my son is failing grade 7, I forgot to put on deodorant this morning, and I don't know what to make for dinner. How are you?"
Nobody is looking for that kind of answer, and few people would be willing to give one like it, so it's easier to simply say, "Fine, thanks," and why not? It doesn't hurt anybody.
In terms of telling the truth, the whole truth and nothing but the truth in creative nonfiction, I can't remember a time when I've ever taken that oath before I started writing. I believe that as long as the main idea is accurately portrayed, the details can be modified. In a work of creative nonfiction, people's appearances can sometimes be changed as long as their actions remain the same, and this can be said for other minor factors like weather, time of day, smells, and even speech. You can play with imagery if it will benefit the essay. Typically when you write, there is an idea, a message, a feeling, and/or an opinion that you try to get across. In order to make it more of an art form as opposed to a toaster oven instruction manual, creative nonfiction writers have the chance to convey those things by making a few alterations. They're not so much lies in this regard as they are enhancements.
When you take a suit in for alterations, the changes that are made to the clothing are done to fit you and your specific body type. The sleeves might be shortened, the waist taken in, or the shoulder pads removed, but in the end, is it not the same suit? It's still the same colour, the fabric is not different, it still has three buttons, except now you look amazing in it and not like an oversized square on legs. The suit has been customized to enhance the features of your body that might have otherwise been overlooked or hidden. This is the kind of thing I consider to be the "creative" in creative nonfiction: you refine the edges of something true to your liking.
The truth is, not everyone can handle the truth, as you might have seen from some of the people who responded to the latest Wikileaks, who encouraged the "elimination" of Julian Assange, the Wikileaks messenger. Sometimes the reactions to the truths being exposed are more revealing than the secrets themselves. Other times it's not the lie itself that is harmful, but the fact that a lie was told in the first place. People lie constantly, and that can be okay, but sometimes it's not. While being careful as to when certain details are enhanced, either by addition or omission, creative nonfiction writers still write about what they feel or know to be true. The key to being a good writer of this genre is knowing when to tell a lie and when not to.