I'm of the opinion that you log the find on the day you sign the log sheet. That said, I feel there's a little leeway that could be given. Such as identifying where the cache is located, but not touching the cache or log sheet (i.e. no signing of the log sheet) and then coming back on the day that you're going to claim the find and actually logging it. It may not be there when you come back or if you're not able to make it out then your streak is snapped.
I will admit that I identified a few LPCs in my area (didn't actually lift the skirts) and one cache were I spotted the container from about 10' away and decided not to make the grab to save them for easy grabs on extremely busy days when working on my 366 Calendar grid. Does that make me a bad cacher or breaker of the rules?