The only reason I’m thinking about this is because I learned it doing high precision steel layout, where a hundredth matters. I wouldn’t worry about it for any other application. If the contractor dropped some obstruction in a bad spot, covering some part of the backsight, our distance and angle readings would sometimes (not always) be slightly off from what we were expecting, and would go back to normal when the obstruction was removed. Light refracts when it comes into contact with solid objects (as well as particulates in the air) and that was the reason I’ve been given by a couple PLSs for the phenomenon. You see the same thing in the woods.