Hey all! Long time reader, first time stander-upper.
I log all my DNF's that I have spent a reasonable amount of time searching for. I look at it as a congratulations to the owner on a job well done. Though I always explain why I felt I didn't find it - i.e. light was fading, muggles in area, really good hide... so there's no concern on the part of the cache owner that something is amiss. Being relatively new to the game I don't log it as an indicator to the owner that something is wrong. If I feel that way, I'll just send the owner an e-mail.
I do have an issue when I go to look for a difficulty 4.5 nano and see that 100 people have had no problems at all finding that cache - I expect to see some DNFs on difficult hides and wonder, when I've been searching for an hour, how come everyone else was able to find it so easily. As a cache owner I think I would rather see people being challenged by a difficult hide that I've put a lot of effort into. And as a cache owner I'd like to know if 5 people in a day looked for my cache, but if they didn't find it, and nobody logged anything, you'd never know anything was up.
But in the end it's up to all of us to play how we see fit.