I've posted this before but it bears repeating...

On one of my early frame builds I cleaned the flux off by mechanical means on the outside and didn't do anything on the inside. Painted it up and rode the heck out of it for a good year or so until noticing corrosion on the bottom bracket around where the seat tube inserted into the shell. Looked closer and found some other corrosion in similar locations on the frame. Concluded that flux was migrating through porosity/pin holes in the brazing filler. Not good. Since then I'm a bit manic about removing flux.