I used to open them before I took them to Gamestop or EB. With FYE I left the games sealed in case they refused it (out of the system or they had too many). No one ever said anything to me.
Even when I sent games into EBGames.com, I opened them. I guess it was 4 years ago they had a deal, trade in any 4 current gen games excluding sports games and get Ninja Gaiden for XBox for free. The cool thing was they sent you a $50 gift code that could be used on anything. I remember going into CC and picking up Nascar 2002 (that wasn't excluded) and some other junky games for under $20 total to mail them in.