I never edit the songs that come out. And they tend to come out as a whole. The closest thing I have ever done to editing them is just cutting out a verse, but never rewriting lyrics.
When we are in pre-production, this is the best job in the world. Working 10 to 7, sitting around and brainstorming with the other writers, making things funnier and writing and rewriting scenes - that's as fun as it gets.