Before I started building my house, I considered stockpiling Romex, since I knew I would need a bunch of it. As it turned out, I bought it as I needed it, and in every case, paid less than when I was thinking about stocking up. If you buy a bunch of copper, what are you going to do with it if prices continue downward?
If that's not what you're asking, well, I plumbed our new house with PEX, and am a big fan of it. People say copper is 'forever', but I had to replace several fittings on our FL house over the years. They simply wore out at the elbows. Cheap fittings? Abrasive water? I don't know, but the plumbing in that house was less than 30 years old.