We, in Team Pitkis, have composed a guideline concerning DNF:s for ourselves and It's quite simple to follow. We do log DNFs in cases we have made a proper attempt to find the cache. That means that if attempt was postponed by pending dusk or something like that, that does not count as a proper attempt.
Instead of logging evey single visit or every single idea of the possible riddle solution are not that interesting material for other cachers. But when the DNF logging is limited to those proper attempts, some hints may be given away about the wrong approaches. Of course the cache owner has his/her say in possible hints, but in general I have personnally found many DNFs to be very illuminating when solving the riddle.
As it was mentioned earlier, those DNFs give the cache owner some feedback concerning the difficulty/terrain appreciation and suggestions on when to time ones maintenance visits and so on.
So summa summarum... I'm very much pro-DNFs and at least we in this team find them very welcome in our own caches.
Aki from Team Pitkis