I would center them.

The photos below look correct to me i.e., the distance between the side of the mirror and a sconce is always the same. The distances between sconces and adjacent walls or doors are ignored.

Bathrooms · More Info

Wyckoff Transitional / Contemporary Master Bathroom · More Info

I would put them 10" away from the mirror. Just to give you a visual, I used one of the pictures above and cut it so the wall left beside the each mirror is 15". The first picture shows the sconces at equal distance on each side of the mirror .i.e:

5" wall + sconce + 10" wall + mirror + 10" wall + mirror + 10" wall + sconce + 5" wall.

The second picture shows the sconce centered on the wall .i.e:

7.5" wall + sconce + 7.5" wall + mirror + 10" wall + mirror + 7.5" wall + sconce + 7.5" wall.

I like the first picture better

Thank you!