cayugad
For me, and myself only - I do not see the need here. Now if I lived in on the Mississippi in the south or if I was along one of the seaboards (salt) I certainly would add the sheath. Shaun - living northern Montana said all he used for storage is the conditioner without a dry patching until next use.
He also did mention if you put oil on top of the conditioner to dry patch before shooting (a regular rifle)
Apparently they have been listening as they are starting to do some experiments with ML's - course he says if you shoot BH - it is not a problem at all - but you should never leave ANY bore unprotected for a length of time.