Sounds reasonable. OK, like they say, "one measurement is worth a thousand expert opinions." We need an experiment. I've got some old wheels sitting around that I never use. Suppose I check the spoke tension with my Wheelsmith tensionometer this weekend, and then check the tension at various intervals over the next couple of years? I really think the amount of stretch will be negligible, especially if you check the percentage elongation specs of the steel used to make spokes. Likely oxidation of the material will have a more significant effect as far as turning a twang into a thud. Otherwise, I would be retensioning the wheels I normally use more often. I'm using two sets of wheels fairly regularly right now, and they haven't been retensioned since I built them last year.
Here's an anecdote: Back in the 1970's I had some track wheels built. They were highly-tensioned and tied and soldered. I stopped using them around 1980. When the indoor Burnaby velodrome opened up around 1995, I started using these wheels again. So the wheels were sitting there for about 15 years. I didn't retension the wheels at all, but they were just as tight as when I left them, and I'm still using them! But then, track wheels will pretty well last forever (no brake track, no rain/dirt/grit). I think they will start breaking spokes when the steel crystallizes/becomes too brittle due to oxidative effects or whatever (I have broken 1 or 2 spokes, so they must be under sufficient tension)
I think we need a metallurgist to chime in here! (what do I know?)
Luis.