I have a piano. I have taken it apart several times for repainting. I change the color of it every now and then. Basically it is a harp, with padded hammers to strike the strings.
I could see how keeping the strings taught would help keep it in tune.
But, on the other hand I could see how not keeping the strings taught could cause no harm.
But on the third hand I could see how not keeping the strings taught could soften/unwind them and make future hold of a tune difficult.
It's you're call. I would get it tuned every 4 or 5 years, just to keep things straight.
Tune it up, it's no different than anything else. A tune up is always good. Plus at your next drunken dinner party when someone plays chopsticks, it sounds better in tune.
If we had room I'd swap you for a beautiful 'primary blue' upright.
