I try to log DNF's on every cache I "do not find" and it doesn't necessarily mean that I think the cache is missing. It means, I gave it a good search and couldn't find it. If I really think it is missing, then I state that in my log. If I look and have to cut my search short for some reason (muggles, weather, etc.), I will log the DNF. The key here is that I have to started my search. If I arrive at the site and decide not to look (for the reasons stated above), then I will just post a note. If I look for a cache, don't find it, but return later that same day and find it, then I will not log a DNF for my first attempt. I will just state in my Found It log that it took me more than one try that day.
As a cache owner, DNF logs and other notes are GREAT feedback about the cache. First of all, they tell me someone is looking for it. I have some caches that I was going to archive because they had not been found in a while and no one had posted a DNF or note against the cache, so I assumed that no one was looking for them. Then, someone would tell me at an event that they had looked for one of these caches and couldn't find it, and not to archive it! So feedback from searchers is very, very good! DNF's are also a good indicator that the cache may be missing. If I get a number of DNF's in a row, it tells me that something may be wrong and that I need to check on my cache.
I really think that many cachers, whether they will admit it or not, feel that a DNF log is a sign of failure and a "bad mark" against them that they aren't good geocachers. Everyone needs to get past this and realize that even the most experienced geocachers can have a bad day of geocaching and experience a lot of DNF's! Personally, I have struggled with some caches that newbies spotted right away! We all have those days!