Dave, you did a masterful job of cutting and assembling this item. As for the backing,personally, I don't like being able to see through cuttings like this one. Otherwise, the background colors won't match the wood colors, and that takes away from the image. If you don't think it needs a backer, did you check to see what it looked like up against the wall without a backer? That is what I'd suggest you do next time, then there will be no doubt if one is needed or not. To sell something like this, I would charge $35.00 at the market. But, if it's a friend that wants to purchase it, then I will work on my price a bit.
Len