Look at just one of the cases - say the leftmost one in the image. If you rotate it 180 degrees, it will look the same. Since you marked the pieces of wood on just one side, you don't have to worry about accidentally flipping the pieces over, just rotating them.
The two pieces should be identical anyways - if out of alignment, the domino will either cut to the left of the mark on both pieces, or to the right. Then you rotate one of the pieces to butt them together. If the domino cuts to the left of the mark, after rotating the piece to put on the bottom, the bottom mark will be even more to the left of the top mark. The center of the domino will be centered between the two marks.
The two pieces should be identical anyways - if out of alignment, the domino will either cut to the left of the mark on both pieces, or to the right. Then you rotate one of the pieces to butt them together. If the domino cuts to the left of the mark, after rotating the piece to put on the bottom, the bottom mark will be even more to the left of the top mark. The center of the domino will be centered between the two marks.