I've seen this topic many times on many different forums, but in scanning through recent posts on this board, I couldn't find a recent discussion.
So, there's pros and cons to both sides of this.
Yesterday I had an unfortunate multiple DNF day. Maybe the caches are still there, maybe they're not. I suppose that's beside the point.
I live in an area where not many people log DNFs, unfortunately. I also live in an area where there are a million micros hidden in high traffic areas and an area where rural areas that have caches hidden are prone to flooding. Also, there are many caches around that haven't been logged in months... pushing years.
I usually log a DNF when I can't find a cache. Sometimes I visit a site more than once before I log a DNF if I feel that there is a reason I couldn't look for the cache properly.
If no one logs a DNF, the cache owner isn't likely to check on their cache.
... but a cache, which may still be there, with a DNF (or perhaps several) may discourage others from going to look for it.
So... what do you think?