As soon as they pay you and it's done, get it out of your world. You don't want responsibility.
Now, on the flip side, given the ease of digital backup, keeping copies is a good way to put some extra bread in your pocket every once in a while when someone calls and says, "Please tell me you still have a copy- we lost what we had." I had this happen recently when a project I only spent a day mixing one song for lost an album's worth of stuff. I had the whole project on my drive and made more than I was paid to mix it "retrieving" it- I told them I'd deleted it but that the drive hadn't been used since and that I might be able to get it back. They were more than happy to pay for what would have been the loss of a month of recording.