I keep all my manuals in a file cabinet.
If I need to be able to regularly refer to the manual - such as for a sewing machine or a small kitchen appliance - I make a copy (or print one from the manufacturer's website) but always leave the original in the file cabinet with all purchase information.
I've sold old stuff on Craigslist and some buyers have told me, "I called on your ad because you had all the owners manuals."