This is a common question which you'll hear as many preferences as there are respondents. It's also a somewhat controversial subject, and some have almost religious convictions
Many photographers adhere to their camera's aspect ratio diligently, these seem to be mainly fine art photographers. Others shoot for a certain end-use® preference, especially those who shoot for magazines. I think those who crop each image differently depending on the end use® are in the minority, though, and most prefer to have just one crop of each image. Reason for that is that even a seemingly trivial crop which doesn't cut any subject matter from the frame might have a big impact in how the image is perceived.
As for me, I'm a brütal cropper: I ignore all format-dependent aspect ratios, and crop to whatever aspect ratio each image "requires," wholly dependent on my eye for composition. I use my cameras' native aspect ratio only occasionally (ie. I crop almost all my shots vertically and/or horizontally), and I almost never use screen or paper aspect ratios. This means there will be black or white edges around my images in whatever form they are presented in, but I'd rather have those than let myself be constrained by set aspect ratios - which to me are artificial, arbitrary and not helpful in producing good images for my purposes.