Perhaps someone can answer this question about tig filler rods based on facts.

I've seen many welders sand the copper coating from the filler rods before they will weld. I've never done this. I can't see where it makes any difference, but maybe I don't have the experiance to see the difference.
They all swear by it, and would never weld without first sanding the copper off. Anyone with a real answer? Oh and what about my 308 rods? I see people sanding those too. I hope I'm not opening a can of worms..