When I put mine together, I couldn't find the thick spacer that I had ordered.   So, since I had a 2 pack of the thinner ones, I used that on both sides.  I found the thick spacer sitting around, so I put it on.  I didn't notice a difference either in noise, temps, fan speed or hashrate.  So if someone was trying to save a few bucks, they get away with using a 2 pack of thinner spacers.