I thickness my FB's to 5mm,
Then slot,
Then shape,
Then glue the FB to the neck,
Spray the finish,
Glue neck on,
Then final sand the FB as I always get a slight hump over the body which has to be removed before fretting.
Some fret before gluing the FB on, some fret after glueing the FB on but before finishing.
Neither is the more correct way- the only correct thing is the end result which is to not have a hump where the neck meets the body.