For lubing threads and o-rings on flashlights which have anodized threads i use nyogel 640g (it's not conductive) and it's ok for that purpose. I also have couple of flashlights on which threads are not anodized. What do you do in that case, to put some grease or leave it dry as it is ? I saw on several flashlights with tail switch that conductivity is achieved not only with threads but also with flat surface at the end of the tube which touch metal plate of the switch. If you take that in mind i assume it's OK to lube threads with non conductive grease but will it be OK in flashlights where currents are about 3A. Then all that 3A will pass through thin surface at the end of the tube.
I first try and grind/trim down the brim of the tailcap, this many times will allow it to sit deeper onto the body tube and bottom the battery tube directly against the switch pill. Its hit or miss though, and often times the design of the parts does not allow the switch pill to bottom-down before the battery tube O-ring gasket crashes into the tailcap threads. I use a caliper to take depth measurements first to make sure it will work. On lights like this I then lube the threads, but keep the battery tube edge clean. If you polish it up clean you can end up with a really nice looking job... better than OEM.
Generally though I run my threads dry when they are required to conduct -B current. I just lube the gasket.