I haven't ever used a lockring or even locktite for that matter. I have a front brake and use it for quick stops for nomal slowing I use leg power and haven't had my cog unscrew yet. I was dicussing it with a friend and we decided that under normal slowing even IF it came loose it would take quite a few revolutions for it to actually come off so you should have time to pedal forward and screw it back on. I think the only way its going to be dangerous is if you are skidding to stop which is something that's way to hard for me anyway...
$0.02