I too, have my Dad's slides. He started in 1938 with DuFay Color. Processed them himself and mounted in glass. They have faded a lot. After that, Kodachrome, and they are still beautiful. I also have 8mm movies from 1939 thru 1989. They are all Kodachrome, and beautiful. Yes, one should project them now and then.
I used to take a lot of slides, but now do mostly B&W. I do have a slide copier that goes on my Nikon so I can copy them to color neg. film, usually Ektar right now. Works fine.