I got this tip from Alex Anderson when she had her tv show. When pinning rows in preparation for machine piecing we usually slip a pin through the stitching of the front and back seams, aligning them so they'll come together perfectly when we're done. Often, however the two pieces are just slightly askew after they're sewn. If this happens to you, try this: Insert a pin, as before, through the stitching of both seams, but then add another pin about 1/16th to the left of the first pin and another pin about 1/16th to the right of the center pin. Then remove center pin. After you've sewn the row, the seams should all go together perfectly.