If I remember, I'll back them off to the lowest setting, but that's about all I do for de-tensioning diamond wires. I don't feel it's that critical, but I may be wrong.

I'd replace them if anything was looking suspect. If a diamond wire fails, chances are it will take the mast with it. On a fresh water boat, the diamond wires should just about last eternity. They are under a pretty constant load, unlike standing rigging which is constantly be shock loaded, and worked through a wide range of tensions from sheet tension, wind ranges, wave state, etcetera, while being used.