While I’ve always concentrated on shooting people and not landscapes, shooting portrait style (vertically) has always been the way to go. You draw the most interest by getting close up, cropping, and filling the frame. So as long as the person is in a vertical position, a portrait frame would best fill the image with the subject. This style of shooting also carried on to my wedding photography for the first few years, always filling the frame. As my work progressed towards photo journalistic style, the images were less about each individual portrait of every person at an event, and more about capturing the entire scene and atmosphere.
A wedding is full of visual stimulation, from the architecture of the church, the colors of a garden, to the lighting of the reception hall, they all need to be part of the photo so the subjects have a setting or environment within the story.
Another reason for shooting landscape mode is reducing the “energy” in the photo. A portrait photo stands tall and has more potential energy than a landscape photo sitting horizontally. Just like you would not feel comfortable watching a vertical movie screen versus a horizontal wide screen. You want to be relaxed while staring at a photo. A landscape photo also simulates a movie scene.
I still remember the immediate transition from 85% portraits to 85% landscape while shooting weddings, and I’ve loved the results even more since then. Whether you still prefer to frame vertically or horizontally, you must be conscious about the environment you are including in the frame. Tight crop is usually safer at the expense of loosing environmental information, but sometimes too much negative space makes the photo boring as well.