cropping is really subjective as well as technical - the rule of thirds works well when you have portraits, etc but not so great at times you have a jumble of stuff. Also, for scrapping, you have all your page elements to draw the eye to your photo so the rule of thirds isn't as important in many cases... you might crop a photo all the way into a face for example.
Cropping also brings up the discussion of photography equipment as well... its really the only time I feel that MP's come into play with a digital camera. In general, the higher the MP, the more you can crop and still get a decent photo.. of course, sharpness is also the key - if the photo isn't sharp to begin with, cropping will certainly magnify that.
A good example is a photo I took of a heron a few years back using a 200-500 zoom with 1.4 teleconverter on it and a 6MP camera. The photo when viewed on the PC normally wasn't bad.. zooming in certainly showed a bit of fuzziness as well as aberations from the atmosphere... when I tried to crop it, whamo - major unsharpness and clearly not my best work. Pulling out some of the sharpening tools I use in photoshop clearly made a huge difference... but couldn't save the photo and I ended up reverting back to the original and just using that for a layout and print.
In this case, I needed a better fixed zoom lens to capture that bird from that distance which would have enabled me to do the kinda of crop I wanted to... also, if I had been using a 10 or 12 MP camera.. I would have had more data to work with for the crop.
So... I guess it is important to keep all this in mind when cropping... but the important thing is to have fun!!
