We've delayed finding and logging a cache (the log in the cache, not online) so that we could log it on a day we needed it.

I have not actively found logged a physical cache log one day and logged online another day. That is not right IMO.
I have suspected where caches were and not actively seeked and logged them until a day I needed a cache.
If a person saw a cache because it was in plain sight (for example a hide-a-key on a light pole but did not log it physically or on line until another day I'd think that was acceptable.

If a person actively seeked out and found caches so they would know where they were on a day he needed a find, I'd think that's a bit over the line. But it really does not affect other cachers a lot so I would not make a big deal about it. If they were doing the same to log the 100 day or 366 challenge they are cheating themselves.

The first cache I found I actually found and opened the day before but did not sign the physical or online log it because I wanted to bring my son there to find it with me.