As most panoramic photographs are of landscapes the obvious choice would be to shoot in landscape mode with your camera. This will allow you to capture the panorama in the least number of shots, however there is one drawback to this approach. It can be extremely difficult to ensure all your shots line up perfectly in the horizontal axis. Consequently when you stitch your photos together you will end up with ragged edges along the top and bottom edges because the individual constituent photographs of the panorama are at slightly different heights. So now you will want to crop your photograph to get rid of these untidy edges and the result is that the overall height of the photograph will be reduced. So the easiest way to get around this issue is to take the shots in portrait mode. Although it needs more shots to get the panorama it does give you much more tolerance of variations in the height of the the constituent photographs, so when you come to crop the final panorama you have much more to work with and don’t lose so much. You can see the difference between two camera orientations after the panorama has been cropped to make a clean edge.
Panorama taken in landscape mode
Panorama shot in portrait mode