I don't know if it is proper or not, but most places I know of put a rectangle in the middle of the line representing the wire and put the wire number in the rectangle.
Lots of places just draw the number right over the wire, which I personally like because it takes a minimum of time and uses a minimum of real estate. But draw it any way you want to, there's at least as many "standards" as there are designers.
Put the number over top of or under the line is the proper way. i have been an electrical maintenance tech for 8yrs went to school to get associate and that is how it is on all prints and books i have seen.
the problem with the rectangle in line is that it could be miss understood as a fuse or other componet and the number to refer to.