You can back up all day long with a walking foot! Even if you decide you do not want to use it for piecing, it is a MUST for the long seams such as strip piecing, adding borders and the binding.
Have you ever sewn 2 pieces of fabric together that are the same length and then when you get done, the top piece is 2" longer than the bottom? How in the world did that happen, right?
The way the walking foot is designed is to pull the top layer of fabric at the same rate the lower piece of fabric is being pulled by the feed dogs. Another name for a walking foot is an even feed foot. In theory all layers go through together.
I do not backtack while piecing unless it is a Y seam, or something that is set in. This is my personal choice and it is NOT wrong to back tack all of the time. It just is not necessary if another line of stitching is going to cross the edge.
I often open seans, turn them in opposite directions where I pull out a stitch or 2. Also I do ALOT of reverse sewing and it is easier if I do not have to deal with reverse stitches.