I don't often log my DNF's. I usually assume they are my own fault, and that no one in general will benifit from hearing about how thick I am.
That said, sometimes you get to a place where you have zero'd in with the gps, taken care to read the previous logs and the hint, and still fail...these will often turn up muggled or missing.
How to tell the difference, it seems to me, is really the question. Logging the former is nice but, well, trivial. Logging the latter potentially saves people loads of time repeating your futile quest, or get the OP to head up some maintenance work.
I dont' have a hard or fast rule, I wish I did. The longer I do this, the better my "geosense" becomes, and the better decisions I make regarding stuff that's broken vs. stuff that I'm failing to find.
Your thoughts appreciated.