Well, to be honest I'm referring to DTP software like QuarkExpress, which stretch the image to fill the box. If you think of widescreen TV's, which stretch the picture to fill the screen, which can make people look fat. On a photo it's not really noticeable, as there are more pixels than on a television screen. As the latest versions of Photoshop include DTP ability then, it should do this without a problem. Problems can arise if you don't make your printer aware of this, as in the process rasterization it can default back to the ratios, as you suggest.
