Location of sconces depends on many things. The mantel height, if combustible material, must meet the code for clearance to the firebox and this differs with how far out the mantel projects. You must know mantel height first. Next is figuring out what goes on and above the mantel. You need to leave room for wall hangings and objects sitting on the mantel. Everything needs to be in proper proportions.

You can stall till finding out more from your interior designer by asking the electrician to tack some boxes at a best guess location and leave the wires a few feet longer than they need to be so that you can adjust later.