Typically you want to align it the way it was growing before, so if it has any noticable curve, you will want that pointing towards the light (up) if it is straight and your mounting it horizontal, I would pick a side that may appear to have been hit with more light. Maybe it has more color one one side, more/raised coralites, etc. Anything that would lead you to believe that side was facing direct light before.
But you do not have to do this, that is just my personal preference is not based on anything other than a assumption that positioning will be "less stressful" to the frag, which it may or may not be.