As a cache owner i would say log your visit asap, whether it be find or a dnf.
Two reasons for this,
If you find it and post a log, i know its still there and in play for all to find (not to mention i quite enjoy reading the stories that go with some finds - "TFTC" tells me its there, but is a bit boring)
Secondly, if you post a DNF, I am likely to see the post and keep an eye on it. I'll even check a cache after one DNF if i am in the area.
Either way, its all good if you log your visit, from a maintenance point of view and a reading about the hunt involved in finding it