Hmm. I'm just reconsidering your question here Jack and wondering if I got hold of the right end of the stick.
You asked if I use new SAE tube nuts when making new SAE flares.
Well, my "problem" - which has turned out not to really be a problem - is that both my tools produce SAE flares so I have no option to make DIN flares. - I'm very attracted to that DIN tool Frost are doing for 30 something quid though.
Your question sounds a bit "horse before cart" if I can say so without giving offence. So, yes, If I'm making an SAE flare and the tube nut required is an SAE (imperial) thread then I will be using new nuts wherever possible (which is pretty much always. However I may be making an SAE flare on a pipe which I'm going to be using a Metric Tube nut with (which Ideally I probably shouldn't be doing, but it works for me for the reasons in my above earlier post) and I would, in this situation, be using a new metric nut. In other words I will always use a new nut if I can get one and would only consider using a second hand nut if a new one is not obtainable and I had a used one which was in very good condition.
I'm not quite sure if that's the answer you were looking for? If not please do rephrase and ask again.