How safe are dental x-rays aimed relatively close to the eye?

Very safe. There is no evidence that dental xrays can affect the eyes. If the tube is aimed near your eye the dental assistant is most likely trying to get an image of an upper tooth. Due to the fact that the eyeball is made up of mostly water the emitted radiation just goes right through with no harmful effects. This is assuming that the unit is set to the proper dose and functioning properly.