If you hope to get your photographs published in magazines, verticals are often preferred because of the dimensions of the pages. With some subjects such as this Hoary Marmot checking things out, horizontal images just don't work. This photo may not make good viewing on a horizontal television screen, but then again that's not why it was taken.
Vertical or horizontal? The best thing it seems is to consider the "rules," perhaps break them, experiment with both formatsand let your instinct and satisfaction guide you.