I tend to log most of my "no-finds" as "no-finds", except in a few special
circumstances.  

If I'm unsure about the location (like, because of a locked gate or
somesuch) and I didn't really look for it that hard, I will sometimes just
write a note.

Also, there have been a few caches that I couldn't find the first time
out.. I didn't log no-finds on the repeated attempts.  Holiday Cache
really gave me a hard time on multiple attempts, but I only logged it as a
"no-find" once.

-Fedl