Personally, I don't worry about the rules, but as I've never managed to write 50K in a month, I've never claimed to have "won", so I don't count it as cheating. I regard it as being more like infiltrating a huge marathon, jogging along with the runners for a while and then slipping quietly away as they sprint for the finish. :)
In other words, to me, NaNoWriMo is a huge wave of writing motivation and in the years I have taken part, I shamelessly rode it to help me to write faster than I normally would. But I don't obsess about the word count. I've written several proper novels before, so I don't see the point in writing drivel just for the sake of adding words to the document. As I can't write 1667 words day after day if I'm going to maintain the quality, I therefore usually challenge myself to do a half-NaNo on whatever project is most convenient.
Having said that, the last NaNo I attempted I did do properly. I started with a brand new story and finished up with 30,000 words or about two thirds of a novel. Since then it has sat for two years untouched, so perhaps what I ought to do this year is finish the draft and sort out the end? Hmmm... That's starting to look like a plan...
Of course November isn't the best of months for me and after months of no deadlines, the first weekend of November will be taken up with the TEFL course and I have an essay due for the OU Worlds of English course on 12 November. However, it's usually the case that the more I have to do, the more I get done because I don't waste time faffing around doing unimportant things, so perhaps I ought to give it a go?
[Cross-posted from Dreamwidth by way of a backup http://heleninwales.dreamwidth.org/70213.html. If you want to leave a comment, please use whichever site you find most convenient. Comments so far: .]