If the faces were custom, and there was a double-backer, then I'd display the front.
There's no need to see card after card of just the back design, when you could see the back design plus 55 other custom cards. If it was a deck like Fantastique (where the back designs are all slightly different), I might choose differently, but I'm currently more in favour of showing the fronts. I mean, look at the Curator deck, or the Fulton's Clip Joint deck. I'd much rather see the faces than the backs.