The higher a ceiling is, the more flexibility you have for a mounting height on the wall. Plus, placement depends on the sconce you choose and whether it is acting as an indirect light source or a decorative element in the room. If the fixtures are decorative, then you can go 6 to 7 feet above the finished floor to the center of the junction box.
The caution here is that some sconces don’t have the junction box connection located exactly in the center — as with the sconces in this photo — so you need to see where the power feed comes. The interior designers I work with will get the sconces and hold them up on the wall prior to hard-wiring the junction boxes so that they can see what looks best aesthetically.
Also, whether a sconce is short and wide or long and narrow greatly affects the placement as well. If you can’t get a hold of the sconces prior to the installation you can cut out in-scale “templates” made of cardboard to use as a guide. This placement challenge is most often the case with exterior lanterns. It is very hard to tell what is the correct mounting height (or proper scale for that matter) until you can look at them from a distance. Plus, often the ground is not level, so the mounting height needs to be measured from the eave height down. If the sconces are more opaque and being used to provide ambient light, then they can mount slightly higher on the wall: 7 feet to 8 feet for a 10-foot ceiling.
Sconces in the bathroom are a different matter. These are used as task lights to evenly illuminate the face for shaving, applying makeup or light spackling. Here, the rule of thumb is to mount them at eye level — usually 5 feet, 6 inches above the finished floor. I tend to recommend taller, narrower fixtures so that all family members, regardless of stature, get adequate task light.
I am not a fan of bath bars over the mirror unless they are used in conjunction with side lights. Alone, they cast hard shadows on people’s faces, making them look older than they are. Plus, the shadowing makes it hard to see your face correctly to do all those things you need to do at the mirror.
And I do have a new book out that does address these issues and more called “Residential Lighting, A Practical Guide to Beautiful and Sustainable Design.”