I encode the credits with a very low quality, but I make sure I can still read them, in case I want to check the name of an actor or a song in the movie. At a quantizer like 26, the video for the credits from the last movie I encoded took only 3 MB, and the audio should not be big either, so I don't think it affects the overall quality of the movie.
Some movies show epilogue at the credits portion, so I always keep it. Sometimes credits shows some behind the scene or something like a blooper, so I encode it with the same quality as the main part.
another vote for lowest quality possible :) I don't even have to be able to read smaller fonts in the credits, they just have to scroll and play the music. Usually, you can get them down to 2 or 3 MB.
Back in my DivX 3 period I would always keep the credits through the cast... I really didn't care who was the assistant to the set painter - but it depends on the film... Not going to try and salvage the credits on a Bruce Lee import... Not going to cut anything from StarWars EP1.