In an analogy to my classic car hobby, the rarer the part, the more I keep on hand. I have 8 spare tail lights for my 1958 Buick Limited (rare part) and two extra rear gravel guard sets (extremely rare), I wont't get rid of them unless a close friend needs one. Common and cheap parts I let go or keep one spare, can always get more later.
Old appliances are different because many parts are rare - who would have thought that people would collect them? I would keep spares only for the machines I own or plan to own, sell the rest if you have no space*. "Expendable" parts (belts, hoses): I buy as I need. These parts often dry rot as they age, so they'll break anyway if used.
* Always seems to bite me in the butt, 'cause I'll get rid of a part for something I think I'll never own and then I'll run across the infamous "deal that can't be beat" and I'll then need the parts I just sold.
Basically, if you have storage, keep'em all. You can always help out friends and AW.org members as the need arises.