I flip up. It is also dependent upon the orientation of the information on the cards. As long as they are placed on the table in the right direction, it may not matter. I would rotate the deck if it was opposite of my preference.
I agree that it will largely depend upon how you have structured them, but my natural inclination is to flip things sideways. With that said, if it's better for the game or makes more sense to be horizontal, I probably wouldn't notice the difference.