Well since I did so well on the first guess I'll have another go.
I have a left hand prop and a RH thread on my packing gland
which would tend to tighten the outer nut by the shaft rotation. So if you have a RH prop maybe go with a LH packing gland if you're paranoid about the shaft rotation loosening the nut.
But I have to say I've never heard of a packing gland nut being loosen by the shaft rotation. In your situation I think I would get the nut that I would remember how to work
a few years down the road when I'm tired, the boat's rocking and water
is gushing in around the shaft I have me head
in the bilge
trying to tighten up the packing while the boat's drifting down on a reef.
Now, waiting for the experts to chime in.