Not necessary to mention in the log, but it is helpful to others (online and logbook).
I have noticed that a great number of Europeans add this to their online log:
IN - (name of trackable)
OUT - (name of trackable)
IN - # of trackables (including 0)
OUT - # of trackables (including 0)
And GG means a count not the tracking number of the trackable.
I think it is a matter of curtesy to the trackable owners, and the cache owners. And it helps in researching missing trackables. For every trackable that I have retrieved or dropped off, I always mentionned its name as retrieved or dropped off in the cache log in addition to the item's own log. I have received severral thank you e-mails from their owners for including that info in the cache's log. And I am now in the habit of mentioning the trackables that I have discovered in a cache. That has now drawn more thank you e-mails from the item owners. It takes very little time for this curtesy to cache and trackable owners. Furthermore, when I see that a trackable is not in a cache, I also mention it in the cache log and the trackable log. Again, more thank you e-mails coming. I don't collect thank you e-mails but that tells me that what I do is appreciated.