If you want to see how a crop will look in the final image but still play with it, set your workspace up like this:

After starting the crop with the C key, adjustments of the crop rectangle in the viewer are accompanied by thumbnail regeneration in the browser. Once it looks right, press A to finish. The same trick works in full screen mode:

Just make sure that the thumbnails are set to be visible all the time by setting the viewer mode to On:

Tags: Tips And Tricks, Aperture, Cropping, Workflow, Workspace, Thumbnails