Probably a good rule of thumb. For some reason I find it hard to shoot in anything but the highest resolution available and I am a JPEG shooter. But I also have never been a mp counter and am still perfectly happy with prints up to 16 x 20 from my ancient D1h at 2.7 mp. I treat the full resolution image like many here do with their raw images. I save the full resolution image as my stored negative, but resize almost everything that I want to use for normal uses. I am also a prolific cropper so that is somewhat of another reason to start with the highest resolution. But with modern digital imaging, you can not only resize smaller, but you can upsize also. I have resized images from my D1H to 24mp and they are stunning. Also, as to combining pixels, the old D1H and D1X nikons did do this as they actually had a 10.8mp sensor and combined pixels to output lower resolutions.