I don't think that real world has 3/2, 4/3, 5/4 or any other crop and I don't think it is natural to let print size decide your crop. Crop to subject.
My point was to take aspect ratio into account when making a lens choice when looking at another format.
For example a photographer that shoots mainly fashion magazines will be working in a crop closer to 5/4.
If that photographer shoots 35mm DSLR he is pretty much always cropping off the top and bottom so the diagonal
he uses to make a focal length conversion should take that into account.