Bottom-to-Top Zipper Sewing Direction
When sewing a conventional zipper application, using a regular zipper (not invisible or separating), it is best to stitch each portion of the zipper from starting at the bottom and moving up. This type of directional stitching helps with two important zipper construction aspects:
The zipper remains better aligned when stitched from the bottom up. When stitching in this direction, any tension and rippling in t eh zipper tape is release at the top which preserves the zipper's bottom alignment along the seam. The bottom of the zipper tape tends to lay a bit flatter which allows for more control at the beginning of the stitching. Sometime's the top zipper tape edge can shift and get caught by the machine needle and presser foot.
When the zipper is stitched in closed position, it is easier to stitch from the bottom moving up as this will keep the zipper slider more comfortably out of the presser foot's way.