You don't need a walking foot for Stitchin' Mission lessons. You'll be able to get along without one, but after machine-sewing binding to a quilt, and knowing it's easier with a walking foot, you'll probably want one.
If you intend to continue making quilts after Stitchin' Mission lessons, a walking foot is a necessity. You'll also use it for straight-stitch machine quilting, and piecing more intricate blocks. I use my walking foot quite frequently and wouldn't be a happy quilter without it. That's why I encourage anyone shopping for a sewing machine, whether new or second-hand, to purchase a walking foot, or a machine with a built-in dual feed/even feed option. Negotiate a walking foot into the price and you won't be sorry.