Garf did some studies on escape size and growth in SPS and discovered (this has been years ago now) that when they were mounted at about an 8:00 position, they encrusted and put out more branches quicker than a vertically mounted frag (and much faster than the mother colony, which is where escape size comes in). I mount mine vertically or off of vertical (sometimes as much as 45-60 degrees), but that is just a preference thing for me.
According to Eric Borneman, mount them horizontally is more natural. It allows the coral to heal quicker by allow water flow and circulation to the fresh cut. And by maximizing the tissue in contact with the mounting substrate allows the coral to start encrusting faster. And as everyone else said it is suppose to increase budding of apex growth stems. But I'm not a SPS guru, and have never really noticed a difference, so I'll let someone else chime in.