I prefer most of my nails to be into the ceiling plane, since there is sometimes shift between walls and ceiling. when framing a house, the framer can install a "nailer" on the side walls, parallel to the joists. this would give a crown moulding installer something on the ceiling plane to nail to, on the walls that are parallel with the joists.
nailing crown to located ceiling joists above is easy on the two walls, but if there is no "nailer" on the other two, I install a backer board first.
in other words, a backer board is used when there is not sufficient nailing material behind (stated above). it can be glued (ceiling and wall surfaces)and nailed into the side wall studs and plate.