I've been told that you're a fool if you don't flow nitrogen when you do your brazing. The idea is that the nitrogen sheild the brazing process on the inside of the refrigrant line so no soot or impurities form, therefore keeping trash out of your refrigrant line.
I wouldn't mind doing a little bench test. Brazing a line and the charging a some with nitrogen and do the brazing. Cut the (2) lines right down the middle long ways and compare the interior.