When I sold my old car, all the old parts from previous visits to the casa were in the trunk. So yeah, I tend to hold on to replaced parts unless they're beyond repair.

I changed steering wheels last year and my stock unit is still in my closet.

The exception is brake pads, which I replaced recently. They're still relatively healthy, with a good layer of brake lining left above the service limit. However, I've read that brake dust is pretty bad for your lungs, so I got rid of mine ASAP.