If you don't need the frame rate, I'd just forget messing with the crop in camera. The results are the same as if you do it later, less the flexibility of doing it later. The cases you cite are static, so one might argue that it really doesn't matter. I shoot sports, and sometimes I just goof it up and get the subject only partially in the frame. If I do that with DX crop, it's a waste case. If I do it with FX (non-)crop but try to fit it into the DX crop area, if I miss, the frame is probably salvageable through the simple expedient of using a different crop. The situation is the same with birds in flight.
I can imagine that saving the storage might be a motivation, especially in the field, but I'd rather just get enough CF/SD and deal with the problem at leisure later. One might argue that doing it repeatedly is a lot of overhead, but I think that it's easy to apply such crops in batches in Lightroom or Photoshop (and I presume most other post processing can do that too). If the situation is so common that you find that it should be the default, my advice is a longer lens or better nature skills, or both.
_____ Brian... a bicoastal Nikonian and Team Member
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