Different cameras have different aspect ratios, and this has various impacts on your photography. For isolated stock photos, I've found that there is a way to increase your use of the sensor. For example, on my Canon 40D it has a 3x2 ratio - 50% longer than it is high. If I'm shooting something long, I often find I can get closer (making the subject larger in the frame) if I shoot it corner to corner in the frame, rather than squared up. Ultimately, this means I get an image out of it that is longer in terms of pixels. In the example below, I did two shots:
1. Key horizontal in frame.
2. Key rotated about 40% so it went corner to corner.
The first obvious thing was that I had to actually move closer with my macro lens to fill the frame when I went corner to corner. Below you can see the result of taking the rotated key and making it straight (right side of image). I ended up with a larger image. Here I've just copped the head of the key to show the difference in size with a 100% crop.
What does this all mean? It means that for an isolated subject you end up with a larger file that normally possible with your camera - say a 12MB file by pixels from a 10MB sensor. I would expect a little loss of something when I rotate, but close inspection of these two images suggests it is more than made up by getting more pixels on each detail in the subject.