I'm in the process of rotating my machines right now. I just added two to the herd and I figured I better give some of the old girls a spin before I get the new ones in to play with.
I also keep an inventory of my machines for my dear daughter who inherits all my worldly goods. My inventory lists each machine, model no., serial number, how much I paid for them, when I bought them, where I bought them, a list of presser feet and accessories, and a brief description of the kind of sewing they do. This way she'll have a pretty good idea of the worth of each machine.