Note : In another post in this topic, someone said that they noticed that Rocco was running around limping. I believe that in the book when Clemenza is contemplating who to use to kill Gatto, it talks about a guy in his ranks named Rocco Lampone and tells about his having a permanent limp as a result of being injured in the war.
DC, you are absolutely right. Rocco was injured during WWII (performing acts for strangers) and was left with a limp. I always wondered why FFC and/or the actor chose not to have him limp until this scene, though. At least I can't remember Rocco limping in any other scene. Unless it's the one where he's eating the Chinese food.
Watch Rocco as he walks to the dock to greet Johnny Ola -- he is clearly limping.
The limp is one of the reasons why I think his assignment to the Roth assassination in the airport was the same kind of deal given to Pentangelli in the "We was like the Roman Empire" scene. The Roth assassination in the airport is a suicide mission, all the more so because the assassin has a limp and therefore has zero chance of getting away. Michael knew that Rocco was one of the traitors, but "kept him close" until the time was right to knock him off.