I have replaced them and also reused them on different Fiestas. I would suggest using only Ford hardware if you are going to replace them. Never ever ever ever ever use hardware from a local hardware store. The Ford fasteners are designed specifically for the application. The shank design is unique, plus the coating and material are also unique. The torque values won't be correct if you don't have the exact coatings. I spent 4 years in suspension engineering and most of that time was working with fasteners.
If you do happen to reuse them, be sure to torque them exactly as specified. I added a little blue Loctite to all of my fasteners. I also added a special product called Torque Seal that you place on the outside of the fastener and it cracks if the fastener moves. This allows for visual inspection of the joint to make sure it is tight. I check mine when I wash the car since the seal is visible from the outside. If you have followed the torque procedure properly and get a loose joint, replace the fasteners. Also, be aware that anti-seize changes the torque. Different types of anti-seize and different types of fastener coatings result in either a lower clamp or higher clamp load based on a given torque.
The best bet is to always use new fasteners and follow the proper procedure.