That's a nice workshop set up you have there John.
I use de ionised water for the salt bath but normal tap water for washing, I was assured the tap water here is good enough but I still like to let it stand overnight to release any traces of chlorine before I use it.
I'd read that copper will kill the bath salts instantly but I'd imagine there's only a slight trace of it in your water, as you say though, distilled water works just fine.
Good luck with the rust blueing, it's good fun when it's working and we'll worth the extra time. A guy brought an old SxS I'd done a few years back for a check over last night, I swear the barrels looked better than when they were freshly done, maybe continued oiling and buffing over time adds to the look
