I run a fixed cog on each side, even when threaded for a freewheel. I don't run a lockring, but I do run a front brake. All my double threaded wheelsets for the fixed gear run fixed/fixed cogs. I don't like to mix fixed and free on the same wheel.

As far as single speeds, I have a conversion that's a dedicated single speed and nothing else. I like taking this bike out on hilly rides so I can coast on long descents.