You dont have to copy your entire Runtime - as the others have pointed out, it will remove all your textures and geometries too. But its only the libraries which are clogging up your workspace. You can copy library folders freely as you suggest, and put them back at will. I do this myself to keep Poser 4 relatively tidy. You need to maintain the basic folder structure, thats all. You can also investigate a utility called Pbooost which does this for you "on the fly".