When I was an apprentice goldsmith, the master caught me back dragging the file. He asked me why I did it, and I told him it somehow made it cut smoother. Wear, on gold, isn't a factor on back dragging because of the metal softness. He told me that I would file faster, more precisely, and more uniformly if I would just learn to file correctly. He showed me how, and I learned, to cut with every stroke deliberately, feeling and seeing how the metal is cutting with the file. He was right. I can file several times as fast, and much more precisely than someone that saws at the work. It did take some learning, and a lot of observation to get a feel for what works.
So, I don't backdrag now, except in a hard to access situation that makes it necessary. But, I never have noticed a file getting dull backdragging. Only from hitting concrete, stone, or sharp edged, hard steel. I also don't think storing files together in a drawer hurts them, but I still keep them one layer deep, side by side, carefully spaced in my user drawers.