Personally I think you should log your DNF if you stopped looking and walked away. The best thing about a DNF is that it lets the cache owner know there may be a potential problem.
Now with that said, there are some very tough finds out there. You have to work a bit for them. DNF's are a very real part of the game. I don't know anybody who has found all of the ones they have looked for. Part of the challenge then becomes going back and resolving the DNF.
Also, on most DNF's on my caches I will write and ask if they want an extra hint. Most cache owners will gladly help out. Why? Because caches are meant to be found!
The bottom line is that the choice is yours.
I'd really rather not cache, but I am helpless in the grip of my compulsion!