Actually I haven't traded in anything to Gamestop in a LONG while, but yeah I used to keep the manual and case to the copies of the Tom Clancy Trilogy I used to trade in during the 'trade 3, get $10 extra' promo for last gen games that GS used to run often.
However, I only did so because otherwise Gamestop would've counted the three game set as ONE game and I would've had to try and find 2 other games to trade with them, thus reducing my profit on each set.
Oh and I did trade in the only copy of Shadow Hearts: Covenant that I found discs only, but that was just to be a jerk not because I would've missed out on any credit had I traded it complete. I still have the 2 disc case, manual and insert SOMEWHERE around the house too. I made a good buck on that trade, since the game traded for nearly $20 then and I paid $3-4 maybe.