I used to blow down the barrel after each shot, mainly because that is what I was taught. I can see no danger unless you dump a fresh charge in before you blow. The only time (we think) when an ember lit the next charge was when a fellow rammed the ball with a fresh load. The explosion shot his ramrod and 20 gauge ball through his hand and the roof of the shooting shed. The thought is, that ramming the ball resulted in a blast of air over a hot ember of some sort, fanning it enough to ignite the powder. Maybe in such a case, blowing down the barrel before loading might have caused the ember to burn out, and this may not have happened. It can be argued many ways. Luckily, all that metal went between his bines, and did little damage, beyond an ugly scar.